Add Module Composer API

[Half-Shot]

Split out from #33166

Part of a series of PRs to extend the Module API for the integrations team.

This one is the smallest change to add the ability to insert text into the composer.

Checklist

• I have read through review guidelines and CONTRIBUTING.md.
• Tests written for new code (and old code if feasible).
• New or updated public/exported symbols have accurate TSDoc documentation.
• Linter and other CI checks pass.
• I have licensed the changes to Element by completing the Contributor License Agreement (CLA)

[t3chguy]

How useful is this Module API really if you don't know what mode your user's composer is in?

one of 4:

1. CIDER with markdown enabled (default)
2. CIDER with markdown disabled (settings)
3. RTE with rich text enabled (labs)
4. RTE with rich text disabled (labs)

Depending on which option the user has this Module API will yield very different results, no?

[Half-Shot]

Let's just make this a plaintext function that's common between all 4 of those kinds. If future module makers want to markdown and go nuts, they can add that functionality. Hopefully in a world where we have fewer composers.

[t3chguy]

The description for the ComposerAPI is to alter the message composer, this method doesn't alter it?

[Half-Shot]

I had other plans for the composer API, so it will be doing some altering, but maybe I can find a more general phrase.