* 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
