Fixes #12573
Fixes #12038

Issue #12344

**Bug**
Current colorization rules are too relaxed. Links inside lists of block quotes for example end up getting colored differently than normal links.

**Fix**
- Add a scope to the textmate markdown grammar so that we can identify the symbol that starts a list or quote.
- Remove text colorization rule for lists and quotes since this just duplicates the normal text color.
- Only apply special puctuator colorization to the start symbol for lists and quotes

Issue #8264

**Bug**
Indented paragraphs are not highlighted in markdown files. Up to three spaces are allowed before the first non space character.

**Fix**
Add rule to consume optional spaces before paragrpah start.

closes #8264

**Bug**
Issue #7725

Nested html is not colorized properly for markdown:

```md
<div class="custom-class" markdown="1">
  <div>
    nested div
  </div>
</div>
```

**Fix**
According to the commonmark spec, general html blocks end with a blank line: http://spec.commonmark.org/0.25/#html-blocks

This change relaxes the while so that we only check for blanklines, instead of trying to match the start tag (which fails when nesting elements)

Issue #8636

**Bug**
Fenced code block closing tags break colorization if they are prefixed with spaces. According to the commonmark spec, up to three spaces are allowed.

**Fix**
Update defintion of fenced code block closing tag to support spaces.

Closes #8636

Issue #12289

**Bug**
Html tags prefixed by 1 to 4 spaces are not colorized properly

**Fix**
Consume optional leading whitespace when matching html tags instead of only starting from previous match location

closes #12289

Adds c# to the list of language highlighted in markdown fenced code blocks. You must have a c# language extension install for this to work.

Closes #7003

Fixes #12648

**Bug**
In Markdown preview, code blocks in headers are the wrong font size.

**Fix**
Specify that code blocks in headers should inherit their font-size, instead of brining their own.

Updated integrated themes indent guide colors

Merge pull request #12472 from mjbvz/show-markdown-preview-for-any-file-type-when-viewed-as-markdown2

Show Markdown Preview Menu Button For any File Opened as Markdown

also use the oppertunity to clean up, migrate to the new list widget etc