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/core/macros.nim b/lib/core/macros.nim
index 57764410e..61b3a7b43 100644
--- a/lib/core/macros.nim
+++ b/lib/core/macros.nim
@@ -182,7 +182,7 @@ template `^^`(n: NimNode, i: untyped): untyped =
 
 proc `[]`*[T, U: Ordinal](n: NimNode, x: HSlice[T, U]): seq[NimNode] =
   ## Slice operation for NimNode.
-  ## Returns a seq of child of `n` who inclusive range [n[x.a], n[x.b]].
+  ## Returns a seq of child of `n` who inclusive range `[n[x.a], n[x.b]]`.
   let xa = n ^^ x.a
   let L = (n ^^ x.b) - xa + 1
   result = newSeq[NimNode](L)