Use W3C's EBNF

The EBNF syntax isn't strictly defined and there exist many variants of it.
This change makes FTL's grammar spec conform to the variant defined by W3C
which they use in the XML and the XQuery spec:

    https://www.w3.org/TR/REC-xml/#sec-notation

Author: stasm

README.md

@@ -4,3 +4,7 @@ Fluent Syntax (FTL)
 Specification, design and documentation for FTL, the syntax for describing
 translation resources in Project Fluent.  FTL stands for _Fluent Translation
 List_.
+
+The grammar is defined using [W3C's EBNF][].
+
+[W3C's EBNF]: https://www.w3.org/TR/REC-xml/#sec-notation

fluent.ebnf

@@ -1,50 +1,49 @@
-body                 ::= (entry NL)* (entry)? EOF;
+body                 ::= (entry NL)* entry? EOF
 entry                ::= comment
-                      | section
-                      | message
-                      ;
+                       | section
+                       | message
 
-comment              ::= '#' .*;
-section              ::= '[[' __ keyword __ ']]';
+comment              ::= '#' char*
+section              ::= '[[' __ keyword __ ']]'
 
-__                   ::= [ \t]*;
-NL                   ::= [\r\n]+;
+char                 ::= [https://www.w3.org/TR/REC-xml/#NT-Char]
+__                   ::= [#x20#x9]*   /* space, tab */
+NL                   ::= [#xA#xD]+    /* line feed, carriage return */
 
-identifier           ::= [a-zA-Z_.?-] ([a-zA-Z0-9_.?-])*;
-variable             ::= '$' identifier;
-keyword              ::= [a-zA-Z_.?-] ([a-zA-Z0-9_.?- ]* [a-zA-Z0-9_.?-])?;
-builtin              ::= [A-Z_.?-]+;
-number               ::= [0-9]+ ('.' [0-9]+)?;
+identifier           ::= [a-zA-Z_.?-] ([a-zA-Z0-9_.?-])*
+variable             ::= '$' identifier
+keyword              ::= [a-zA-Z_.?-] ([a-zA-Z0-9_.?- ]* [a-zA-Z0-9_.?-])?
+builtin              ::= [A-Z_.?-]+
+number               ::= [0-9]+ ('.' [0-9]+)?
 
-member-key           ::= number | (identifier '/')? keyword;
-member               ::= '[' member-key ']' __ pattern NL;
-default-member       ::= '*' member;
-variants-list        ::= NL (__ member)* __ default-member (__ member)*;
-traits-list          ::= variants-list | NL (__ member)+;
+member-key           ::= number | (identifier '/')? keyword
+member               ::= '[' member-key ']' __ pattern NL
+default-member       ::= '*' member
+variants-list        ::= NL (__ member)* __ default-member (__ member)*
+traits-list          ::= variants-list | NL (__ member)+
 
-message              ::= identifier __ '=' __ (pattern | pattern traits-list | traits-list);
+message              ::= identifier __ '=' __ (pattern | pattern traits-list | traits-list)
 pattern              ::= unquoted-pattern
-                      |  quoted-pattern
-                      ;
-unquoted-pattern     ::= (unquoted-text | placeable | block-text)+;
-quoted-pattern       ::= '"' (quoted-text | placeable)* '"';
-unquoted-text        ::= ([^{] | '\{')+;
-quoted-text          ::= ([^{"] | '\{' | '\"')+;
-block-text           ::= NL __ '|' unquoted-pattern;
+                       | quoted-pattern
+unquoted-pattern     ::= (unquoted-text | placeable | block-text)+
+quoted-pattern       ::= '"' (quoted-text | placeable)* '"'
+unquoted-text        ::= ([^{] | '\{')+
+quoted-text          ::= ([^{"] | '\{' | '\"')+
+block-text           ::= NL __ '|' unquoted-pattern
 
-placeable            ::= '{' __ expression __ '}';
+placeable            ::= '{' __ expression __ '}'
 expression           ::= quoted-pattern
-                      |  number
-                      |  identifier
-                      |  variable
-                      |  select-expression
-                      |  member-expression
-                      |  call-expression
-                      ;
-select-expression    ::= expression __ ' ->' __ variants-list;
-member-expression    ::= identifier '[' keyword ']';
-call-expression      ::= builtin '(' __ arglist? __ ')';
-arglist              ::= argument (__ ',' __ arglist)?;
+                       | number
+                       | identifier
+                       | variable
+                       | select-expression
+                       | member-expression
+                       | call-expression
+
+select-expression    ::= expression __ ' ->' __ variants-list
+member-expression    ::= identifier '[' keyword ']'
+call-expression      ::= builtin '(' __ arglist? __ ')'
+arglist              ::= argument (__ ',' __ arglist)?
 argument             ::= expression
-                      |  keyword-argument;
-keyword-argument     ::= identifier __ ':' __ quoted-pattern;
+                       | keyword-argument
+keyword-argument     ::= identifier __ ':' __ quoted-pattern