feat(common/core/desktop): add debug events

Adds debug events to the kmx processor and corresponding unit tests.
This corresponds largely to the way that keyman32 emits debug events,
but with one key change: debug events in keyman32 are processed
synchronously, whereas in Keyman Core, the full set of debug events will
be returned alongside the final action list for the key event.

Given this difference, there will likely need to be some more data
returned in individual debug events, specifically around intermediate
context manipulation, as required for step-by-step debugging. However,
that will come in a subsequent PR.