Add support for VimWiki default syntax

[ludenticus]

Problem
VimWiki is a personal wiki for (neo)vim that has its own default syntax (.wiki), which is a little bit different from markdown. LspLog in neovim sends a warning: "Unsupported code language ID 'vimwiki', treating text as plaintext".

• :tag: and :tag:tag are Ok, but :tag: :tag: is diagnosed as "incorrect spaces".
• _word1 word2 word3_ (for italics): word1 and word3 are not recognized.
• {{{ code }}} and `code` are diagosed as text
• *bold* is Ok
• [[link|description]]: link is diagnosed as text

Solution
Add support to VimWiki default syntax (.wiki)

Context
VimWiki .wiki syntax is similar to .md, but its basic markup has some peculiarities:

= Header1 =
== Header2 ==
=== Header3 ===
*bold*
_italic_
[[wiki link]]
[[wiki link|description]]
:tag: 
`code`
{{{
  code
}}}