feat(js): add schema-based per-field nullable support for xlang

[theharsh999]

Why?

JavaScript xlang schema-based struct serialization currently treats all fields as nullable and always writes a per-field null flag.
This introduces unnecessary overhead when a field is known to be non-nullable by schema design.

What does this PR do?

• Adds schema-based per-field nullable support for JS xlang struct serialization.
• Preserves backward compatibility by treating fields as nullable by default.
• When nullable: false is explicitly specified:
  • Skips writing the per-field null flag.
  • Throws a clear runtime error if the field value is null or undefined (includes field name).
• Extends StructTypeInfo field schema typing to allow nullable?: boolean.
• Adds a minimal unit test verifying:
  • Non-nullable fields throw on null.
  • Fields without nullable keep existing behavior.

Related issues

Does this PR introduce any user-facing change?

Yes.
This PR introduces an opt-in schema-level configuration (nullable: false) for JS xlang serialization.
Existing schemas are unaffected unless nullable: false is explicitly set.

• Does this PR introduce any public API change?
• Does this PR introduce any binary protocol compatibility change?

Benchmark

This change is opt-in and only affects schema-based fields explicitly marked as nullable: false.
No performance regression is expected for existing schemas.