Add C-API helpers for embedders: property accessors, exception, promise

[andreasrosdal]

Summary

Adds eight header-only inline helpers to quickjs.h that collapse
patterns repeated dozens of times in real embedders. Motivated by an
audit of the Nordstjernen browser's QuickJS binding (src/js.c,
~10k lines), where the same five-to-twelve-line dances appear at
hundreds of call sites.

I am making a web browser here:
https://github.com/nordstjernen-web/nordstjernen

This PR was created using Claude.

New helpers:

Helper	Replaces
JS_FreeCStringSafe	if (s) JS_FreeCString(ctx, s);
JS_GetPropertyStrInt32	get + null/undef check + JS_ToInt32 + free
JS_GetPropertyStrInt64	"
JS_GetPropertyStrFloat64	"
JS_GetPropertyStrBool	get + null/undef check + JS_ToBool + free
JS_GetExceptionAsString	JS_GetException + JS_ToCString + "stack" fetch + format
JS_ResolvePromise	call resolvers[0] + free value + free both resolvers
JS_RejectPromise	call resolvers[1] + free reason + free both resolvers

Each helper:

• Takes ownership uniformly so callers can't leak a JSValue.
• Silently clears any pending exception raised by an internal
  conversion (so embedders don't accidentally propagate a coercion
  error from a getter).
• Returns a sensible fallback for missing / null / undefined
  properties.

Why

JS_GetException + JS_ToCString + JS_GetPropertyStr("stack") is
the single most-repeated pattern in the Nordstjernen binding —
appearing in every timer, every microtask drain, every event
dispatch, every fetch callback. It's also where refcount bugs hurt
most, because it runs on the error path. Folding it into one call
makes that path both shorter and safer.

The typed property accessors target options-bag reads (timer
options, fetch() init, event init, IntersectionObserver options,
etc.), which embedders implement by hand today.

Compatibility

• All additions are static inline in quickjs.h.
• No new exported symbols.
• No changes to quickjs.c.
• ABI unchanged; existing builds keep working without recompile.

Out of scope (follow-ups identified in the audit)

• Per-call interrupt-deadline scoping (JS_SetContextDeadline).
• Atom interning for hot DOM property names.
• A callback-style JS_ObjectForEach for own-property iteration.
• Moving polyfill text-encoding helpers (TextEncoder/Decoder)
  into C.

Each touches more of the engine and warrants a separate PR.