RichText: Mark onSetup, getSettings as unstable APIs

[aduth]

This pull request seeks to deprecate the following two props of RichText, while still making them available as intentionally-flagged "unstable" equivalent props:

• getSettings
  • Now available as unstableGetSettings
• onSetup
  • Now available as unstableOnSetup

These changes follow a similar pattern to that of #7029 (unstableOnFocus), where the intent is to discourage use of these props, which are already undocumented. Their existence is undesirable, but necessary in current implementations of core blocks. Until the core blocks can be refactored to avoid them, they are marked as unstable to discourage other developers from considering adoption.

These props are considered undesirable because they pierce the abstraction RichText is intended to serve atop TinyMCE. Respectively they are intended to allow a consumer to override TinyMCE settings or to receive the initialized TinyMCE instance.

Testing instructions:

Verify that there are no regressions in the behavior of the List and Table blocks.

Then revert a9a6792 in your local copy of the branch to reintroduce the deprecated prop usage. Verify that the blocks still operate as expected, but log a deprecated warning in the process.

[ellatrix]

Great!

[gziolo]

We still need to update deprecations docs page.