[RichTextBlock][.NET] Update existing richtextblock implementation to match new specification for 1.2

[shalinijoshi19]

1. To address the length concerns, we’re simplifying the payload
   a. Removing the concept of paragraphs, so it’s just a set of inlines
   b. Syntax sugar for auto-converting a text string into a basic TextRun
2. For the wrap/maxLines
   a. Remove those properties
3. For the markdown inside runs
   a. Don’t support markdown
4. For being able to use italics within runs (since no markdown)
   a. Possibly add an “italicize” property to TextRun
5. For fallback support/extensibility (which markdown doesn’t support)
   a. We’ll add that in 1.3 for text runs (too costly to add now)

Changes…

1. Removed “paragraphs”. Now it directly goes to “inlines”
2. Syntax sugar for converting a text string into a basic TextRun object (to reduce payload size when you don’t need any special formatting)
3. Removed “wrap” and “maxLines” properties
4. Removed markdown support from TextRun
5. Added “italicize” (name pending) to TextRun so you can italicize text

[shalinijoshi19]

@RebeccaAnne FYI