Win32: Support Korean input

[cbracken]

This change fixes a bug in Korean input whereby WM_IME_COMPOSITION
messages of type GCS_RESULTSTR were assumed to end composing mode.

This change breaks out an additional handler for "commit composing text"
events. In Japanese/Chinese IMEs, these events typically occur on
selection of a candidate from the candidates list and are mostly
synonymous with an "end composing" event.

In Korean text input, there is no candidates list, but rather a
character is built up as keypresses are handled, and committed as soon
as the character is unambiguously complete; in other words, when either
space/return is pressed or a keypress is received that cannot be
interpreted as a modification of the character being composed and
therefore must be the first keystroke of a new character. In these
cases, we want to commit the previous character without ending the
composition.

To illustrate with an example:

1. User focuses on a text field and sets input mode to Hangul.
2. User presses 'ㄱ'. Composing region contains 'ㄱ'.
3. User presses 'ㅏ'. Composing region is updated to '가'.
4. User presses 'ㄴ'. Composing region is updated to '간'.
5. User presses 'ㅏ'. Result string '가' is committed. Composing region is
   updated to '나'.
6. User presses 'ㄷ'. Composing string is updated to '낟'.
7. User presses 'ㅏ'. Result string '나' is committed. Composing region is
   updated to '다'.
8. User presses space or enter. Result string '다' is committed.
   Composing is ended.

On a non-Korean QWERTY keyboard the following key mappings serve to
perform the above input:

• r -> ㄱ
• k -> ㅏ
• s -> ㄴ
• e -> ㄷ

To support the above, we break out a separate "commit composing" method
and commit on WM_IME_COMPOSITION events of type GCS_RESULTSTR and end
composing on WM_IME_ENDCOMPOSITION events. Further, we eliminate the
workaround in the GCS_RESULTSTR handler for continued composition on
Chinese/Japanese IMEs now that we're no longer ending composition on
that event type.

Fixes flutter/flutter#74812.

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[clarkezone]

LGTM, don't think this one impacts WinUWP target