check if we are in a string

Our existing heuristic looked for a quote _somewhere_ before the
candidate and a quote (doesn't need to be the same type of quote)
_somewhere_ after the candidate.

While this is an okay assumption, it also has flaws, because this means
that the `outline` in the following example is considered inside of a
string:

```js
function foo({ a = "", outline = true, b = "" }) {
  //
}
```

With this change, we will do a little bit of parsing and figure out if
we are in a string (by looking at balanced quotes).

Author: RobinMalfait

packages/@tailwindcss-upgrade/src/codemods/template/is-safe-migration.ts

@@ -3,7 +3,6 @@ import type { DesignSystem } from '../../../../tailwindcss/src/design-system'
 import { DefaultMap } from '../../../../tailwindcss/src/utils/default-map'
 import * as version from '../../utils/version'
 
-const QUOTES = ['"', "'", '`']
 const LOGICAL_OPERATORS = ['&&', '||', '?', '===', '==', '!=', '!==', '>', '>=', '<', '<=']
 const CONDITIONAL_TEMPLATE_SYNTAX = [
   // Vue
@@ -141,8 +140,8 @@ export function isSafeMigration(
   }
 
   // Heuristic: Require the candidate to be inside quotes
-  let isQuoteBeforeCandidate = QUOTES.some((quote) => currentLineBeforeCandidate.includes(quote))
-  let isQuoteAfterCandidate = QUOTES.some((quote) => currentLineAfterCandidate.includes(quote))
+  let isQuoteBeforeCandidate = isMiddleOfString(currentLineBeforeCandidate)
+  let isQuoteAfterCandidate = isMiddleOfString(currentLineAfterCandidate)
   if (!isQuoteBeforeCandidate || !isQuoteAfterCandidate) {
     return false
   }
@@ -213,3 +212,38 @@ const styleBlockRanges = new DefaultMap((source: string) => {
     ranges.push(startTag, endTag)
   }
 })
+
+const BACKSLASH = 0x5c
+const DOUBLE_QUOTE = 0x22
+const SINGLE_QUOTE = 0x27
+const BACKTICK = 0x60
+
+function isMiddleOfString(line: string): boolean {
+  let currentQuote: number | null = null
+
+  for (let i = 0; i < line.length; i++) {
+    let char = line.charCodeAt(i)
+    switch (char) {
+      // Escaped character, skip the next character
+      case BACKSLASH:
+        i++
+        break
+
+      case SINGLE_QUOTE:
+      case DOUBLE_QUOTE:
+      case BACKTICK:
+        // Found matching quote, we are outside of a string
+        if (currentQuote === char) {
+          currentQuote = null
+        }
+
+        // Found a quote, we are inside a string
+        else if (currentQuote === null) {
+          currentQuote = char
+        }
+        break
+    }
+  }
+
+  return currentQuote !== null
+}