refactor(core): split context API from Core primary API

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