Fix IME input when composing in the middle of the textarea

[ntomoya]

Fixes #5456

When screenReaderMode is enabled, moving the caret inside existing text and then committing IME input can cause stale trailing text from the textarea's previous state to be re-sent.

This PR fixes composition range tracking in two places:

• At compositionstart, it uses textarea.selectionStart/selectionEnd (the actual caret/selection) instead of textarea.value.length.
• At finalize time, it keeps support for characters typed immediately after compositionend while avoiding re-sending trailing text that already existed before composition started.

This keeps composition diffing aligned with the real insertion point and prevents stale suffix insertion.

Repro

1. Enable screenReaderMode.
2. Type 一二 via IME.
3. Move the caret between 一 and 二.
4. Commit 一 via IME.

• Before: text could become 一二二 (the previous trailing 二
  is re-sent).
• After: text becomes 一一二.

Validation

• Reproduced with screenReaderMode: true before the fix; no repro after the fix.
• Confirmed no behavior change in normal mode (screenReaderMode: false).
• Manually verified caret movement + IME insertion in the VS Code integrated terminal scenario.

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Related VS Code issues:

• Behaviour when entering Japanese in the terminal. microsoft/vscode#255725
• Character insertion issues in the VS Code built-in terminal due to non-English IME. microsoft/vscode#274992

[Tyriar]

Repro before:

After: