fix: add option to disable sync mode

[Matthiee]

Doesn't fully fix #30 but drastically reduces memory being retained in QuickJSRuntime.

I added 2 test cases to demonstrate this.

Table with memory increases after 100 iterations:

	Sync enabled (default)	Sync disabled
Return object	1320 kB	23.8 kB
Return promise	1791 kB	23.8 kB

[rot1024]

Thank you. Sorry for late reply. I will check your PR.

[rot1024]

@Matthiee CI fails. Is it OK to ignore this error?

FAIL  src/memory.test.ts > memory > memory leak promise
AssertionError: expected 23.8876953125 to be +0 // Object.is equality
 ❯ src/memory.test.ts:103:49
    101| 
    102|     console.log("Allocation increased %d", memoryAfter - memoryBefore);
    103|     expect((memoryAfter - memoryBefore) / 1024).toBe(0);
       |                                                 ^
    104| 
    105|     arena.dispose();

  - Expected   "0"
  + Received   "23.88[76](https://github.com/reearth/quickjs-emscripten-sync/actions/runs/7740470830/job/21105489605?pr=31#step:6:77)953125"

[Matthiee]

@Matthiee CI fails. Is it OK to ignore this error?

FAIL  src/memory.test.ts > memory > memory leak promise
AssertionError: expected 23.8876953125 to be +0 // Object.is equality
 ❯ src/memory.test.ts:103:49
    101| 
    102|     console.log("Allocation increased %d", memoryAfter - memoryBefore);
    103|     expect((memoryAfter - memoryBefore) / 1024).toBe(0);
       |                                                 ^
    104| 
    105|     arena.dispose();

  - Expected   "0"
  + Received   "23.88[76](https://github.com/reearth/quickjs-emscripten-sync/actions/runs/7740470830/job/21105489605?pr=31#step:6:77)953125"

Hi @rot1024,

The goal here would be to have no memory retained hence I put the expected value at 0. However, I was unable to achieve this. I already created this draft PR to demonstrate some ways of reducing the memory when not using the syncing capabilities of the library.

There is still 23.8 kB of memory being retained somewhere, I'm just not sure where exactly...

I was hoping that you could also have a look to see if maybe there is a better way of doing this or if I'm missing something here.

More info on my original issue can be found here #30

Kind regards,
Matthias.