Adds new speech bubble opacity sliders to the accessibility menu. (#35346)

Adds new speech bubble opacity sliders to the accessibility menu. (#35346)

* Adds new accessibility slider for speech bubble text opacity.
Adds new accessibility slider for speech bubble background opacity.
Adds new Cvars to track speech bubble text and background opacity settings.

* Adds a separate option slider for the opacity of the speaker's name on speech bubbles.

* Changes text and speaker default opacity to 100%, as it was before.

* Apply suggestions from code review

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Co-authored-by: slarticodefast <161409025+slarticodefast@users.noreply.github.com>