feat: Add interfaces for focus management

[BenHenning]

The basics

• I validated my changes

The details

Resolves

Fixes #8772
Fixes #8773
Fixes part of #8770

Proposed Changes

Introduces the necessary base interfaces for representing different focusable contexts within Blockly. The actual logic for utilizing and implementing these interfaces will come in later PRs.

Reason for Changes

These interfaces provide the foundation for introducing a central system for managing focus (see #8770 for more context).

Test Coverage

N/A -- This is an interface-only change.

Documentation

Probably not--the class documentation should be sufficient.

Additional Information

See also google/blockly-keyboard-experimentation#142 (comment) for broader technical design context on the introduction of these new interfaces.

[BenHenning]

PTAL @gonfunko.

Also, will the circular import dependency here actually work? I didn't see any errors during build or in the playground, but since the new interfaces aren't actually being used (only exported) I wasn't sure if such errors would actually come up (I'd expect them to be caught during JavaScript generation from the TypeScript code, but I suppose there might be reasons it's ignored).

[gonfunko]

LGTM, the import should be fine since it's just importing the type AFAIK.

[BenHenning]

Thanks @gonfunko!

Will need to hold on merging this since the branch needs to be rebased onto v12.

[BenHenning]

Probably doesn't need a re-review. There was one conflict in core/blockly.ts due to IFlyoutInflater but it was a trivial resolution. The changes look clean and otherwise identical to the previous version that was based off develop.