feat(core): add deleted_context to km_core_actions struct

Fixes #10530.

Adds the deleted_context member to the km_core_actions struct, and
associated unit tests. Simplifies integration by providing the consumer
with all the data they need in order to execute the transform,
specifically around number of delete operations required, without
needing to query the target application context again. The number of
delete operations will vary according to application compliance and
selected encoding; for example a UTF-16 string may require 2
delete-backs for surrogate pairs in the text buffer for a compliant app,
whereas there will be a single delete-back key event for a non-compliant
app.

The deleted_context member should also be used for debug assertions.