[Chore] Reorganize engine.ts

[CharlieFRuan]

This PR reorganizes engine.ts to keep it relatively more organized.

• We move getToolCallFromOutputMessage() from engine.ts to support.ts since it does not depend on any engine-related fields and is stateless
• We move asyncLoadTokenizer() from engine.ts to cache_util.ts for the same reason
• In future, we can implement a common helper for chatCompletion() and completion()
• For engine.ts, we break down the functions into sections:

0. Setters and getters
1. Model/pipeline loading and unloading: reload(), unload()
2. Underlying auto-regressive generation functions: _generate(), asyncGenerate(), interruptGenerate()
3. High-level generation APIs: generate() (deprecated), chatCompletion(), completion()
4. WebGPU info-querying helpers. Arguably, this can be implemented in support.ts as well. Currently these methods are in the MLCEngineInterface, and goes through the WebWorkerMLCEngine - MLCEngine message passing workflow
5. Low-level APIs that interact with pipeline