Several gin_helper code paths run on every native event emit, every IPC
message, every option-dictionary parse, and every C++ -> JS callback.
This change removes a handful of avoidable per-call costs from those
paths.

EmitEvent / CallMethodWithArgs:
- Internalize the event name and the "emit" method name with
  gin::StringToSymbol so V8 dedupes them against its string table and
  property lookups can compare by pointer identity instead of hashing a
  fresh kNormal string on every emit.
- Reuse the emitter object as the node::CallbackScope async resource
  instead of allocating a throwaway v8::Object per emit.
- Drop the redundant inner EscapableHandleScope; EmitEvent / CustomEmit
  already open one immediately around the call.

gin_helper::Dictionary:
- Internalize property keys via gin::StringToSymbol (matching what
  gin::DataObjectBuilder and gin::ObjectTemplateBuilder already do).
- Replace the unconditional Has() + Get() pair in Get() with a single
  GetRealNamedProperty() lookup — one V8 boundary crossing per key
  instead of two, with the same missing-vs-undefined disambiguation.
  Native interceptor objects (process.env, etc.) are detected with the
  zero-cost HasNamedLookupInterceptor() hidden-class flag and routed to
  the Has()/Get() path that drives the interceptor.

gin_helper::Locker:
- Store v8::Locker inline in std::optional instead of heap-allocating it
  per construction; this is on the path of every C++ -> JS callback.

blink converters:
- char16_t::FromV8: read the UTF-16 code unit directly instead of
  round-tripping V8's internal UTF-16 through UTF-8 and back.

Microbenchmark medians (Linux x86-64 testing build):

  EmitEvent (1 listener)              2806 ns -> 1816 ns  (-35%)
  EmitEvent (no listeners)            2788 ns -> 1800 ns  (-35%)
  Dictionary::Get (per key, all hit)  82.2 ns -> 26.9 ns  (-67%)
  Dictionary::Get (per key, all miss) 55.6 ns -> 18.5 ns  (-67%)
  Dictionary::Set (per key)           121  ns -> 82.6 ns  (-32%)
  gin_helper::Locker ctor/dtor        94.3 ns -> 9.85 ns  (-90%)
  C++ -> JS callback (RepeatingCb)    463  ns -> 301  ns  (-35%)

Co-authored-by: Sam Attard <sattard@anthropic.com>