Clean up functions when disposed

[justjake]

• Re-works function binding in newFunction to use a different proxying strategy based on a new abstraction, HostRef. HostRef allows tracking references of host values from guest handles. Once all references to the HostRef object are disposed (either in the host, or GC'd in the guest), the host value will be dereferenced and become garbage collectable.
• Functions passed to newFunction are interned in the runtime's HostRefMap until dereferenced.
• HostRef is also exposed directly for use from QuickJSContext, see newHostRef, toHostRef, unwrapHostRef in the docs.
• Added QuickJSContext.newConstructorFunction, such functions will have their this set to the newly constructed object, although they may also return a new object.

Fixes #223

[Copilot]

Pull Request Overview

This PR cleans up function entries upon disposal by removing them from the internal map and reusing freed function IDs via a freelist instead of always incrementing the counter.

• Extended heapValueHandle to accept an extra disposal callback.
• Updated newFunction to pull IDs from fnIdFreelist and schedule freeFunction on handle disposal.
• Introduced fnIdFreelist storage and freeFunction method for recycling IDs.

Comments suppressed due to low confidence (2)

packages/quickjs-emscripten-core/src/context.ts:1331

• Add a JSDoc comment for fnIdFreelist to explain its role in recycling function IDs upon disposal.

  protected fnIdFreelist: number[] = []

packages/quickjs-emscripten-core/src/context.ts:608

• Add unit tests to verify that IDs from fnIdFreelist are correctly reused when disposing and creating new functions.

    const fnId = this.fnIdFreelist.pop() ?? ++this.fnNextId

[Copilot]

Extract the magic number 8 used in the bit shift into a named constant (e.g., ID_MAP_SHIFT_BITS) to clarify intent.

Suggested change

	const map_id = fn_id >> 8
	const map_id = fn_id >> QuickJSContext.ID_MAP_SHIFT_BITS

[Copilot]

After deleting the last function in a map, consider removing the empty fnMap entry from fnMaps to avoid retaining unused maps.