Relates to #9999.

Relates to #9999.
Fixes #10384.

The context API endpoints should no longer be considered as part of the
standard Core API. The only consumers that have a need to access these
APIs are the IMX integration in Engine for Windows, and the Keyman
Developer Debugger.

These symbols are currently used by Developer:
* `km_core_context` struct
* `km_core_context_type` enum
* `km_core_context_item` struct
* `KM_CORE_CONTEXT_ITEM_END` macro
* `km_core_state_context()`
* `km_core_context_set()`
* `km_core_context_clear()`

These symbols are currently used by Windows IMX:
* `km_core_context` struct
* `km_core_context_type` enum
* `km_core_context_item` struct
* `KM_CORE_CONTEXT_ITEM_END` macro
* `km_core_context_items_dispose()`
* `km_core_context_item_list_size()`
* `km_core_state_get_intermediate_context()`

The following functions and symbols are moving to
keyman_core_api_context.h:
* `km_core_context` struct
* `km_core_context_type` enum
* `km_core_context_item` struct
* `KM_CORE_CONTEXT_ITEM_END` macro
* `km_core_state_context()` function
* `km_core_state_get_intermediate_context()` function
* `km_core_context_set()` function
* `km_core_context_clear()` function
* `km_core_context_get()` function
* `km_core_context_items_from_utf16()` function
* `km_core_context_items_from_utf8()` function
* `km_core_context_items_to_utf8()` function
* `km_core_context_items_to_utf16()` function
* `km_core_context_items_to_utf32()` function
* `km_core_context_items_dispose()` function
* `km_core_context_length()` function
* `km_core_context_append()` function
* `km_core_context_shrink()` function
* `km_core_context_item_list_size()` function

Relates to #9999.

Relates to #9999.

Establishes functions, unit test sources, normalization entry point and
an effectively no-op unit test for normalization support.

Fixes #9999.

Note TODO items:
- [ ] Renormalize cached_context across action boundary. Blocked by #10369.
- [ ] Add extra tests for surrogate pairs
- [ ] Move set_context_from_string into helper module
- [ ] if we don't apply normalization, we still need to fixup the
      app_context, to keep it coherent with cached_context (or at least
      we need to verify that app_context is never used in this
      situation)

…nto feat/core/9999-normalize-output

Per discussion in #10422, we can assume that input cached_context is
always NFD. However input actions->output may not start at a
normalization boundary, so we still need to backtrack to a normalization
boundary in order to get our NFC output. But cached_context never need
change.

This makes no change to the algorithm, but tweaks some of the unit tests
to adhere to this input assumption.

Note: we could consider adding a debug assertion that cached_context is
NFD.

…ssors

Fixes #10421.