Simplify JSON, XML de/serialization in TypeScript

[mristin]

We factor out repeated boilerplate in the generated TypeScript de/serialization code into shared runtime helpers, reducing generated code size and fixing a couple of genuinely redundant computations.

JSON parsing:

• checkIsIterable(jsonable) replaces the null/typeof/Symbol.iterator checks that were duplicated at every list-typed property setter.
• checkIsJsonObject(jsonable) replaces the null/Array.isArray/typeof checks duplicated at every concrete class's and interface's fromJsonable function.
• checkModelType(modelType, expected) replaces the required/mismatch check duplicated once per class with a model type.

XML serialization:

• writeVElement/writeClassElement push directly into the shared parts array instead of each call site inlining openTag/content/closeTag, without introducing any extra string concatenation or array allocation compared to the original code.

XML parsing:

• nextPropertyOpenTag(cursor, className) replaces the ~25-line token-read/end-of-stream/close-tag/wrong-token-kind/namespace-check preamble that was duplicated in every class's property-parsing loop. It returns the token, null, or a DeserializationError directly, so it adds no allocation over the original inlined code despite running once per property parsed, not just once per instance.
• Property close-tag consumption now reuses the already-computed propertyLocalName instead of redundantly recomputing localNameOfTag(propertyStartTag.tag).