[Markdown] support lists in blockquotes, code fences, multiline inline code and multiline emphasis

[keith-hall]

This PR simplifies a few contexts, removes a few incompatible regexs (mainly \G) and adds support for lists in blockquotes, plus includes a few more tests.

[keith-hall]

I've got this to the state where the only incompatible regex patterns are two \Gs :)
Note that I have scoped .begin and .end which could affect color schemes - I can remove these changes if needed.

[FichteFoll]

I suggest this instead:

contexts:
  raw:
    - match: (`+)
      scope: punctuation.definition.raw.begin.markdown
      push:
        - meta_scope: markup.raw.inline.markdown
        - match: \1(?!`)
          scope: punctuation.definition.raw.end.markdown
          pop: true
        - match: '`+'
        - match: ^\s*$\n?
          scope: invalid.illegal.non-terminated.raw.markdown
          pop: true

This allows multi-line inline raw sequences again and isn't explicit about the number of backticks allowed, which is arbitrary. I don't know if this actually represents how markdown parsers work, but some testing here on github seemed to confirm this behavior.

Should also exit on empty lines. See:

test `test

test`

[keith-hall]

@FichteFoll thanks, I have just pushed a new commit with your proposal :)

[keith-hall]

I've done some testing at http://spec.commonmark.org/dingus/ and made the backtick handling more consistent with how that works. This includes support for code fences (though it doesn't highlight the language contained inside the fence yet, I'll leave that for another PR, as I wonder if it would give us more with_prototype recursion limit warnings).

[keith-hall]

Performance comparison (24 KiB syntax test file):

This PR:

Syntax "Packages/Markdown/Markdown.sublime-syntax" took an average of 5.0ms over 10 runs

Before this PR:

Syntax "Packages/Markdown/Markdown.sublime-syntax" took an average of 7.2ms over 10 runs

[keith-hall]

Performance update & comparisons against some Markdown syntaxes on Package Control, for the syntax test file in this PR, which is now 40 KiB:

Syntax "Packages/Markdown/Markdown.sublime-syntax" took an average of 10.4ms over 10 runs (this PR)

Syntax "Packages/Markdown/Markdown.sublime-syntax" took an average of 11.2ms over 10 runs (before this PR - I guess the smaller difference now is due to all the added constructs it lexes in this PR)

Syntax "Packages/Markdown Extended/Syntaxes/Markdown Extended.sublime-syntax" took an average of 12.3ms over 10 runs

Syntax "Packages/MarkdownHighlighting/Markdown.sublime-syntax" took an average of 12.8ms over 10 runs

Syntax "Packages/MarkdownEditing/Markdown (Standard).tmLanguage" took an average of 11.3ms over 10 runs

Syntax "Packages/MarkdownEditing/Markdown.tmLanguage" took an average of 12.9ms over 10 runs

[wbond]

Thank you very much for all of this work!