Add ability to create custom TextInput based components

[acoates-ms]

Currently you cannot create a custom TextInput component.

We'll need to create native APIs to be able to create and customize TextInput behaviors.

[chrisglein]

Some partners have a custom text input that implements rich text and exposes a bunch of custom events for rich embedded format (Example, rich support for "@" tagging). As is TextInput is effectively "sealed". There is some need of native functionality. Or at least this is how partners solved this on Paper. Might be possible to solve as a JS component, but there's desire to share code and integrate with TextInput. A substantial of the code would be shared. Can't be generic because needs the ability to link out to specific services for the partner code.

Recommend working @acoates-ms and partners to ensure the design meets their needs.

There are scenarios here that would be relevant for various messaging apps around \ and @ handling, spellcheck. Think Slack, Teams, etc.. Many scenarios in addition to Office.