Add syntax color to ocamldoc markup

[dlesbre]

A few ocamldoc markup had syntax color, but most didn't. This PR adds color to most markup, using the similar markup in markdown for the textmate scope. For reference, I used the following:

• ocamldoc syntax reference: https://v2.ocaml.org/manual/ocamldoc.html#s%3Aocamldoc-comments
• odoc syntax reference: https://ocaml.github.io/odoc/odoc_for_authors.html
• odoc differences with ocamldoc: https://ocaml.github.io/odoc/ocamldoc_differences.html

Changes

Before this PR, only few constructs had syntax coloring:

• @tags
• [inline code] (note that inline code is rendered using ocaml's syntax highlighting, and not semantic highlighting. Depending on theme, the colors may be very different from the rest of the file).

I added the following:

• {b bold] {1 heading] {i italic] {e emph} (colored the same as their markdown equivalents)
• {^ exponent} {_ index}, likely not colored in most themes (named markup.exponent.ocamldoc)
• {m math} and {math math-block}, odoc extension, colored the same as latex math (which is support.class for some reason...)
• {[ocaml code block]}
• {v verbatim v} and {@lang[ other language code block]} (odoc extension), colored as markdown code block
• {{: link}link-text}
• {!reference} and {!type:reference], as well as {{!reference}alt-text}. I'm not really satisfied with the later though, as I can't color both reference as ocaml code and alt-text as markup (tm grammar don't have a begin/middle/end sadly).
• Lists both simple (leading -) and complex {ul {li item}}
• Target specific formatting {%latex: latex code%} and {%html: html code %} as well as others (rendered as verbatim code)
• tags specific syntaxes for @raise Exn, @raises Exn, @param id, @see "document", @see 'file' and @see <url>

I haven't added the following:

• {C center}, {L left} and {R right} (not supported by odoc)
• {!indexlist} (not supported by odoc)
• html tags (not supported by odoc)

Test comment

Here is a comment showing off the new color highlights that can be used to test them with different themes:

(** This is a test documentation comment:
  @see <https://v2.ocaml.org/manual/ocamldoc.html#s%3Aocamldoc-comments>
       for ocamldoc syntax reference
  @see <https://ocaml.github.io/odoc/odoc_for_authors.html>
       for odoc (implementation of the reference, plus a few extensions, like math)

  Basic markup:
    {1 heading}
    {b bold} {i italic} {e emph} {m math}
    {^ exponent} {_ index} (these are maked, but not colored by most themes)
    {v verbatim v}
    {C center} {R right} {L left} (There are unmarked and uncolored)
    {math
      math block
    }

  Tags:
    @tag
    @raise Exn description    @raises Exn description
    @param id description
    @see 'filename'           @see "document-name"       @see <link>

  Code:
    [let foo x = x]
    {[
      let foo x = x
    ]}
    Other language are an odoc extension. I haven't added any language specific
    code, so this python snippet isn't highlighted as python, only as verbatim.
    {@python[
      def hello() -> None:
          print("hello world!")
    ]}

  References and links:
    [foo] {!foo} {!module:Foo} {!modules: Foo Bar}
    {{: https://example.com}Example link}
    {{!foo}The foo function}
    {{!module:Foo} the Foo module}

  Lists:
    - simple
    - unorderd list
    + simple
    + numbered list
    {ul {- full}
        {li list {b with bold text} }}
    {ol {- ordered}
        {li list}}

  Target specific blocks:
    {% here is some latex \foo $1$ %}
    {%latex: some more latex $x = x+1$ %}
    {%html: <p>hello</p><a href="https://example.com">example link</a> %}
*)

[mnxn]

Thank you for the very detailed PR. The highlighting changes look great!

I only have a few implementation comments:

[dlesbre]

I've also added a small change to fix #1014, (simply removing the global scope name comment.other.ocamldoc from the ocamldoc.json. It won't changes the color of comments in .ml/.mli files since its already added in ocaml.json, but it removes the color from mld files.

[mnxn]

Nice work!