Fix handling of final abbreviations

Previously, if an abbreviation ended the paragraph (for example, if
`Id.` was passed in), the full-stop was seen as the terminal marker
of the sentence and not part of the word (even though it’s a
recognised abbreviation).

Related to retextjs/retext-contractions#6.

Author: wooorm

lib/plugin/merge-prefix-exceptions.js

@@ -10,7 +10,7 @@ module.exports = modifyChildren(mergePrefixExceptions);
  * abbreviation. */
 var ABBREVIATION_PREFIX = new RegExp(
   '^(' +
-    '[0-9]+|' +
+    '[0-9]{1,3}|' +
     '[a-z]|' +
 
     /* Common Latin Abbreviations:
@@ -31,39 +31,46 @@ var ABBREVIATION_PREFIX = new RegExp(
  * sentence ends with a certain word. */
 function mergePrefixExceptions(child, index, parent) {
   var children = child.children;
+  var period;
   var node;
   var next;
 
-  if (
-    children &&
-    children.length !== 0 &&
-    index !== parent.children.length - 1
-  ) {
-    node = children[children.length - 1];
+  if (children && children.length > 1) {
+    period = children[children.length - 1];
 
-    if (node && toString(node) === '.') {
+    if (period && toString(period) === '.') {
       node = children[children.length - 2];
 
       if (
         node &&
         node.type === 'WordNode' &&
-        ABBREVIATION_PREFIX.test(
-          toString(node).toLowerCase()
-        )
+        ABBREVIATION_PREFIX.test(toString(node).toLowerCase())
       ) {
+        /* Merge period into abbreviation. */
+        node.children.push(period);
+        children.pop();
+
+        /* Update position. */
+        if (period.position && node.position) {
+          node.position.end = period.position.end;
+        }
+
+        /* Merge sentences. */
         next = parent.children[index + 1];
 
-        child.children = children.concat(next.children);
+        if (next) {
+          child.children = children.concat(next.children);
 
-        parent.children.splice(index + 1, 1);
+          parent.children.splice(index + 1, 1);
 
-        /* Update position. */
-        if (next.position && child.position) {
-          child.position.end = next.position.end;
-        }
+          /* Update position. */
+          if (next.position && child.position) {
+            child.position.end = next.position.end;
+          }
 
-        /* Next, iterate over the current node again. */
-        return index - 1;
+          /* Next, iterate over the current node again. */
+          return index - 1;
+        }
       }
     }
   }

test/fixture/combining-tie-character.json

@@ -237,6 +237,22 @@
                       "offset": 22
                     }
                   }
+                },
+                {
+                  "type": "PunctuationNode",
+                  "value": ".",
+                  "position": {
+                    "start": {
+                      "line": 1,
+                      "column": 23,
+                      "offset": 22
+                    },
+                    "end": {
+                      "line": 1,
+                      "column": 24,
+                      "offset": 23
+                    }
+                  }
                 }
               ],
               "position": {
@@ -245,22 +261,6 @@
                   "column": 22,
                   "offset": 21
                 },
-                "end": {
-                  "line": 1,
-                  "column": 23,
-                  "offset": 22
-                }
-              }
-            },
-            {
-              "type": "PunctuationNode",
-              "value": ".",
-              "position": {
-                "start": {
-                  "line": 1,
-                  "column": 23,
-                  "offset": 22
-                },
                 "end": {
                   "line": 1,
                   "column": 24,

test/fixture/combining-tie-under-inverted.json

@@ -237,6 +237,22 @@
                       "offset": 26
                     }
                   }
+                },
+                {
+                  "type": "PunctuationNode",
+                  "value": ".",
+                  "position": {
+                    "start": {
+                      "line": 1,
+                      "column": 27,
+                      "offset": 26
+                    },
+                    "end": {
+                      "line": 1,
+                      "column": 28,
+                      "offset": 27
+                    }
+                  }
                 }
               ],
               "position": {
@@ -245,22 +261,6 @@
                   "column": 26,
                   "offset": 25
                 },
-                "end": {
-                  "line": 1,
-                  "column": 27,
-                  "offset": 26
-                }
-              }
-            },
-            {
-              "type": "PunctuationNode",
-              "value": ".",
-              "position": {
-                "start": {
-                  "line": 1,
-                  "column": 27,
-                  "offset": 26
-                },
                 "end": {
                   "line": 1,
                   "column": 28,