tts: markdown and emoji filters + advanced preprocessing

[simllll]

Description

implements markdown and emoji filters
based on python versoin: https://github.com/livekit/agents/commits/main/livekit-agents/livekit/agents/voice/transcription/filters.py

also adds some advanced preprocessng for german and english, based on livekit/agents#3927 (but not landed in main python yet). the advanced transfomfers are not applied by default, but you can optin e.g. by using

voice: {
ttsTextTransforms: getRecommendedTTSTransforms("de")
}

default of markdown + emoji is applied (like in python)

Changes Made

• added transcriptions folder with implementation and tests
• hooked in generation to the tts pipeline and applies the transfomrations (same as in python)

Pre-Review Checklist

• Build passes: All builds (lint, typecheck, tests) pass locally
• AI-generated code reviewed: Removed unnecessary comments and ensured code quality
• Changes explained: All changes are properly documented and justified above
• Scope appropriate: All changes relate to the PR title, or explanations provided for why they're included
• Video demo: A small video demo showing changes works as expected and did not break any existing functionality using Agent Playground (if applicable)

Testing

• Automated tests added/updated (if applicable)
• All tests pass
• Make sure both restaurant_agent.ts and realtime_agent.ts work properly (for major changes)

Additional Notes

---

Note to reviewers: Please ensure the pre-review checklist is completed before starting your review.

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Latest commit: 7418080

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[toubatbrian]

Hey @simllll, the PR is great! However, could you break this down into multiple sub-PRs stacked on each other? That’ll make it much easier to review. Thanks!