Markdown, CommonMark: add support for wiki links. [API change]

Adds the Markdown/CommonMark extensions `wikilinks_title_after_pipe` and
`wikilinks_title_before_pipe`. The former enables links of style `[[Name
of page|Title]]` and the latter `[[Title|Name of page]]`. Titles are
optional in both variants, so this works for both:
`[[https://example.org]]`, `[[Name of page]]`.

The writer is modified to render links with title `wikilink` as a
wikilink if a respective extension is enabled.

Pandoc will use `wikilinks_title_after_pipe` if both extensions are
enabled.

Closes: #2923

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