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/pure/memfiles.nim

@@ -377,7 +377,7 @@ proc `$`*(ms: MemSlice): string {.inline.} =
   copyMem(addr(result[0]), ms.data, ms.size)
 
 iterator memSlices*(mfile: MemFile, delim = '\l', eat = '\r'): MemSlice {.inline.} =
-  ## Iterates over [optional `eat`] `delim`-delimited slices in MemFile `mfile`.
+  ## Iterates over \[optional `eat`] `delim`-delimited slices in MemFile `mfile`.
   ##
   ## Default parameters parse lines ending in either Unix(\\l) or Windows(\\r\\l)
   ## style on on a line-by-line basis.  I.e., not every line needs the same ending.