nimdoc: fix char literal tokenization (#25576)

This fixes highlighter's tokenization of char literals inside
parentheses and brackets.

The Nim syntax highlighter in `docutils/highlite.nim` incorrectly
tokenizes character literals that appear after punctuation characters,
such as all kinds of brackets.

For `echo('v', "hello")`, the tokenizer treated the first `'` as
punctuation because the preceding token was punctuation `(`. As a
result, the second `'` (after `v`) was interpreted as the start of a
character literal and the literal incorrectly extended to the end of the
line.

See other examples in the screenshot:
<img width="508" height="266" alt="Screenshot 2026-03-04 at 16-09-06
_y_test"
src="https://github.com/user-attachments/assets/94d991ae-79d2-4208-a046-6ed4ddcb5c34"
/>

This regression originates from a condition added in PR #23015 that
prevented opening a `gtCharLit` token when the previous token kind was
punctuation. Nim syntax allows character literals after punctuation such
as `(`, `[`, `{`, `:`, `;`, or `,`, of course. The only case mentioned
in the manual explicitly that actually requires special handling is
stroped proc declaration for literals (see the [last paragraph
here](https://nim-lang.github.io/Nim/manual.html#lexical-analysis-character-literals)):

```nim
proc `'customLiteral`(s: string)
```

This PR narrows the conditional to not entering charlit only after
backticks.

lib/packages/docutils/highlite.nim

@@ -326,7 +326,8 @@ proc nimNextToken(g: var GeneralTokenizer, keywords: openArray[string] = @[]) =
       pos = nimNumber(g, pos)
     of '\'':
       inc(pos)
-      if g.kind != gtPunctuation:
+      let followsBacktick = pos >= 2 and g.buf[pos - 2] == '`'
+      if not followsBacktick:
         g.kind = gtCharLit
         while true:
           case g.buf[pos]
@@ -338,6 +339,8 @@ proc nimNextToken(g: var GeneralTokenizer, keywords: openArray[string] = @[]) =
           of '\\':
             inc(pos, 2)
           else: inc(pos)
+      else:
+        g.kind = gtPunctuation
     of '\"':
       inc(pos)
       if (g.buf[pos] == '\"') and (g.buf[pos + 1] == '\"'):