Markdown and HTML widgets

Create out-of-the-box parsers for handling basic HTML and Markdown content, especially for Compose. Other platforms have their own mechanisms for handling such input text, but Compose doesn't have anything natively.

Alternatively, create an out-of-the-box Syntax for HTML or Markdown that can be installed into any parser. Would be nice to have #6 done so that this Syntax can reference the common tags for building itself

Start with the parser and Syntax concepts described in the issue, and review issue #6 for the shared tags it may provide. Decide whether Compose support belongs in dedicated HTML and Markdown parsers or reusable Syntax definitions; done means a working out-of-the-box path for basic HTML and Markdown content.

Written by the indexing model from the issue text.