MarkdownTextBlock: Rewrite lists to be text-based

[jcoc611-microsoft]

Before, Lists were implemented using InlineUIContainer and grid layouts. This looks great, but breaks selection and accessibility. This PR rewrites lists all-up to use a combination of Paragraph, the list bullet character (either a "dot" or the index number/alpha/roman), and smart use of margins and indents.

WinUIRenderer keeps track of lists in a stack fashion, and MyParagraph uses the info from the renderer to offset the text and prepend the bullet character(s).

[michael-hawker]

I love the idea of translating lists into text instead of custom UI, makes a lot of sense and resolves a lot of issues with accessibility and copying.

Seeing how this works, I think the crux of the issue is that a List is contained in a Paragraph and we need to modify the properties of the parent paragraph to make this work/look good.

I'm wondering if we can be better here about using parsing to detect this or having the List Renderer make/be a paragraph vs. having to connect state between the paragraph and lists between the renderer. @nerocui any insights here?

I'd have to look at the parsing tree here (I wrote a tool for that actually a few weekends back, but still need to publish it.) Like I just wonder if in all cases a list is just encapsulated by a paragraph and any text before/after is a separate paragraph. If so, we should just make a dedicated "BulletedParagraph" type which will just output the final result that he Listrenderer does vs. it being a thing which needs to coordinate with the general paragraph.