Fix composition logic for Firefox
Fix composition logic for Firefox

Fixes: https://github.com/microsoft/vscode/issues/106392

In Firefox when inserting Emoji's using the macOS emoji input or using the compositor for characters with accents è, ë, î, etc. the character is inserted twice. Additionally, when adding an accented character the base character was also inserted.

Examples:

🏅 inserts 🏅🏅
è inserts eèè

This change does two things:

- It removes `this._onType.fire(typeInput);` from the compositionupdate event listener. If you are in the middle of a composition there is no need to fire off the event. This change fixes the unaccented character from being inserted. `è` no longer inserts `eè`.

- In the composition end function `browser.isFirefox` is added to the conditional that resets this._textAreaState to the value of the text area. This is needed as the insert of the composition is handled by L294. The textarea needs to be updated correctly when the compositions ends.

Tested in Firefox (Windows 10, Ubuntu, macOS), Safari (macOS), Chrome (macOS), Edge Legacy (Windows 10), Edge (Windows 10)
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