feat(core): ldml processor touch support 🙀

@mcdurdin

@mcdurdin this is somewhat of an umbrella ticket. Trying to capture the discussion in issues for planning.

Support Touch Layouts in LDML.

Waiting on Joshua's 📜 merges
WASM working well, but need more WASM API endpoints into Core
main blocker: Q: how to implement core into KMFW?
Divergence in other platforms
May be able to do some test scaffolding for touch (prior to KMFW intergration)
Q: how to link to processEvent in a lightweight way but without breaking predictive text?
Note: Core in KMFW is not in v17.
🙀 spec(core): LDML KeyboardProcessor 🙀 #5015

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