Veercodeprog/feat/5144 boa class plain properties

[Veercodeprog]

feat(macros): add #[boa(js_init)] support to #[boa_class]

Closes #5144 (partial — instance-side initialization only)

Summary

Introduces the #[boa(js_init)] attribute to the #[boa_class] macro,
allowing embedders to hook into object_constructor without having to
hand-write a full impl Class block. This enables setting own data
properties on a freshly constructed JsObject instance — the first
concrete step toward the plain-property support discussed in #5144.

Motivation

Currently, #[boa_class] only exposes data_constructor via
#[boa(constructor)]. If you want to set own JS properties after the
object exists (e.g. instance.upcast().set("label", ...)), you have to
drop out of the macro entirely and write impl Class manually. The
object_constructor hook already exists on the Class trait for
exactly this purpose — this PR just wires it up through the macro.

Changes

core/macros/src/class.rs

• Parse #[boa(js_init)] on at most one function per #[boa_class] impl block.
• Validate the expected shape: no self, no generics or async, exactly
  three parameters (instance, args, context), return type must be
  (), JsResult<()>, or omitted (normalized to ()).
• Emit fn object_constructor(...) in the generated impl Class when a
  js_init function is present; otherwise fall back to the no-op trait
  default.
• Exclude js_init-annotated functions from method/static registration
  so they are not accidentally exposed as JS-callable methods.

core/macros/src/lib.rs

• Add #[boa(js_init)] to the boa_class attribute documentation with
  a brief usage example.

tests/macros/tests/class.rs

• New Widget class and boa_class_js_init_sets_own_property test:
  #[boa(constructor)] stores the native id field; #[boa(js_init)]
  sets an own data property label from a second constructor argument,
  verifying end-to-end that instance-side initialization works.

Scope

Covered by this PR:

• Instance-side property initialization via the existing
  object_constructor trait hook.

Out of scope / follow-up work for #5144:

• Prototype and static property declarations
  (#[boa(property)] / #[boa(static_property)] → ClassBuilder::property).
• Declarative const-style properties, particularly where values involve
  non-const types such as JsValue.
• Unifying data_constructor and object_constructor into a single
  constructor model (if the project pursues that direction).
• Proc-macro-level type validation for js_init parameters — wrong types
  still produce a compile error at the forwarded Self::… call site.

Testing

# Run the new macro integration test
cargo test -p boa_macros_tests --test class

# Lint check
cargo clippy -p boa_macros --all-targets -- -D warnings