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

tests/stdlib/trst.nim

@@ -1173,7 +1173,7 @@ suite "Warnings":
       lastParagraph
       """
     var warnings = new seq[string]
-    let output = input.toAst(warnings=warnings)
+    let output = input.toAst(rstOptions=preferRst, warnings=warnings)
     check(warnings[] == @[
         "input(3, 14) Warning: broken link 'citation-som'",
         "input(5, 7) Warning: broken link 'a broken Link'",
@@ -1199,7 +1199,7 @@ suite "Warnings":
         rnParagraph
           rnLeaf  'here'
           rnLeaf  ' '
-          rnRef
+          rnRstRef
             rnLeaf  'brokenLink'
       """)
     removeFile("other.rst")
@@ -1558,7 +1558,7 @@ suite "RST inline markup":
 
   test "no punctuation in the end of a standalone URI is allowed":
     check(dedent"""
-        [see (http://no.org)], end""".toAst ==
+        [see (http://no.org)], end""".toAst(rstOptions = preferRst) ==
       dedent"""
         rnInner
           rnLeaf  '['
@@ -1606,6 +1606,19 @@ suite "RST inline markup":
           rnLeaf  'end'
         """)
 
+  test "Markdown-style link can be split to a few lines":
+    check(dedent"""
+        is [term-rewriting
+        macros](manual.html#term-rewriting-macros)""".toAst ==
+      dedent"""
+        rnInner
+          rnLeaf  'is'
+          rnLeaf  ' '
+          rnHyperlink
+            rnLeaf  'term-rewriting macros'
+            rnLeaf  'manual.html#term-rewriting-macros'
+      """)
+
   test "URL with balanced parentheses (Markdown rule)":
     # 2 balanced parens, 1 unbalanced:
     check(dedent"""