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

1 files changed, 1 insertions, 1 deletions

diff --git a/lib/packages/docutils/dochelpers.nim b/lib/packages/docutils/dochelpers.nim
index a11f2bbbb..b85e37983 100644
--- a/lib/packages/docutils/dochelpers.nim
+++ b/lib/packages/docutils/dochelpers.nim
@@ -82,7 +82,7 @@ proc toLangSymbol*(linkText: PRstNode): LangSymbol =
   ##
   ## This proc should be kept in sync with the `renderTypes` proc from
   ## ``compiler/typesrenderer.nim``.
-  assert linkText.kind in {rnRef, rnInner}
+  assert linkText.kind in {rnRstRef, rnInner}
 
   const NimDefs = ["proc", "func", "macro", "method", "iterator",
                    "template", "converter", "const", "type", "var",