Clean up functions when disposed

[justjake]

When disposed, delete the function from the internal map and add the function id to a "freelist" that will be re-used for the next function created. Only increment ID if no freelist id is available.

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.