Serialization Followup work

[joheredi]

Here are some things that were left out from the initial Serializer PR that we need to address:

• [ ] Remove any casting on dictionary deserialization (reference)
• [ ] Only generate serializeHelper.ts if used
• [ ] Fix Date deserialization on Dictionaries (reference)
• [ ] Revisit if we want to generate anonymous models leveraging TCGC's generated name
• [ ] Revisit how we preserve extra properties in serialization (reference)
• [ ] General revision of any casts on serialization
• [ ] Go through property serialization and make sure we are handling null/undefined consistently and correctly (reference)
• [ ] Settle on a single source for Usafge flags (reference)