Implement Pandoc Markdown concise link extension (#20304)

* Implement Pandoc Markdown concise link extension

This implements https://github.com/nim-lang/Nim/issues/20127.
Besides reference to headings we also support doing references
to Nim symbols inside Nim modules.

Markdown:
```
Some heading
------------

Ref. [Some heading].
```

Nim:
```
proc someFunction*() ...

... ## Ref. [someFunction]
```

This is substitution for RST syntax like `` `target`_ ``.
All 3 syntax variants of extension from Pandoc Markdown are supported:
`[target]`, `[target][]`, `[description][target]`.

This PR also fixes clashes in existing files, particularly
conflicts with RST footnote feature, which does not work with
this PR (but there is a plan to adopt a popular [Markdown footnote
extension](https://pandoc.org/MANUAL.html#footnotes) to make footnotes work).

Also the PR fixes a bug that Markdown links did not work when `[...]`
section had a line break.

The implementation is straightforward since link resolution did not
change w.r.t. RST implementation, it's almost only about new syntax
addition. The only essential difference is a possibility to add a custom
link description: form `[description][target]` which does not have an
RST equivalent.

* fix nim 1.0 gotcha

lib/packages/docutils/rstast.nim

@@ -49,7 +49,10 @@ type
     rnCitation,               # similar to footnote, so use rnFootnote instead
     rnFootnoteGroup,          # footnote group - exists for a purely stylistic
                               # reason: to display a few footnotes as 1 block
-    rnStandaloneHyperlink, rnHyperlink, rnRef, rnInternalRef, rnFootnoteRef,
+    rnStandaloneHyperlink, rnHyperlink,
+    rnRstRef,                 # RST reference like `section name`_
+    rnPandocRef,              # Pandoc Markdown reference like [section name]
+    rnInternalRef, rnFootnoteRef,
     rnNimdocRef,              # reference to automatically generated Nim symbol
     rnDirective,              # a general directive
     rnDirArg,                 # a directive argument (for some directives).
@@ -110,7 +113,7 @@ type
                               ## auto-numbered ones without a label)
     of rnMarkdownBlockQuoteItem:
       quotationDepth*: int    ## number of characters in line prefix
-    of rnRef, rnSubstitutionReferences,
+    of rnRstRef, rnPandocRef, rnSubstitutionReferences,
         rnInterpretedText, rnField, rnInlineCode, rnCodeBlock, rnFootnoteRef:
       info*: TLineInfo        ## To have line/column info for warnings at
                               ## nodes that are post-processed after parsing
@@ -281,7 +284,7 @@ proc renderRstToRst(d: var RenderContext, n: PRstNode, result: var string) =
     inc(d.indent, 2)
     renderRstSons(d, n, result)
     dec(d.indent, 2)
-  of rnRef:
+  of rnRstRef:
     result.add("`")
     renderRstSons(d, n, result)
     result.add("`_")