Add support for markdown -> XAML parsing (#17585)

This adds support to the Terminal for parsing Markdown to XAML. We're
using https://github.com/github/cmark-gfm as our parser, so that we can
support the fullness of github-flavored markdown.

The parser parses the markdown to produce a `RichTextBlock`, which
covers just about all the scenarios we need. Since we're initially just
targeting using this for "Release notes", I didn't implement
_everything_ in markdown[^1]. But headers, bold & italic, unordered
lists, images, links, code spans & blocks - all that works. We can work
on additional elements as we need them. The parser is encapsulated into
`Microsoft.Terminal.UI.Markdown.dll`, so that we won't load it on
startup, only when the pane is actually made the first time.

To test this out, I've added a `MarkdownPaneContent` pane type on
`x-markdown` (the `x-` is "experimental"). Go ahead and add that with:

```json
{ "command": { "action": "splitPane", "type": "x-markdown" } }
```

That's got the ability to load arbitrary MD files and render them. I
wouldn't call that experience finished though[^2][^3](and it probably
won't be in 1.22 timeframe). However, it is an excellent testbed for
validating what we do and do not support.

We'll use the markdown parser Soon<sup>TM</sup> for the What's New
panes.

* Done in pursuit of displaying release notes in the Terminal.
* Doesn't quite close out #16495 
* Should make #8647 possible
* may help with #16484

[^1]: the most notable gap being "block quotes" with `>`. I don't think
I can draw a vertical line in a rich text block easily. Footnotes are
also missing, as well as tables.
[^2]: I say it's not finished because the aforementioned MD gaps. Also
the UX there is not polished at all.
[^3]: I don't believe we'll have time to polish out the pure markdown
pane for 1.22, but what the parser covers now is more than enough for
the release notes pane in time for 1.22

.github/actions/spelling/allow/allow.txt

@@ -5,6 +5,7 @@ breadcrumbs
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.github/actions/spelling/allow/apis.txt

@@ -43,6 +43,7 @@ DONTADDTORECENT
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.github/actions/spelling/excludes.txt

@@ -104,6 +104,7 @@
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+^NOTICE.md
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