Add GC callbacks, and a mechanism to fix the threshold

[KaruroChori]

For context: saghul/txiki.js#516 & saghul/txiki.js#520

I implemented a draft of some new core features in quickjs to expose more of an interface for the GC to its runtime. This way it is easier for the application embedding quickjs to control when GC events can run and some related side effects.
This additional control is helpful in the context of systems with some real time requirements.

The extended interface basically consists of:

• A getter for the cg threshold to complete its interface. added in a separate PR
• A flag to fix the threshold, disabling the autoscaling feature which is currently forced.
• Callbacks before and after the CG event (not to be run for manually triggered ones).
  The before callback has a boolean return value to further specify if the event should be allowed to run or not.

Tests are not provided yet since they might be a bit tricky to write, but if there is consensus on this or some alternative variant I will add some.

[saghul]

In general, I'm supportive of the approach.

I didn't comment on the style issues since they are not relevant just yet ;-)

@chqrlie thoughts?

[KaruroChori]

Sadly style is about what you can expect by someone deeply unfamiliar with the codebase and who does not generally work with C :D.
In my defense I was unable to find specific guidelines in the repo, and the naming conventions are not always consistent across the codebase, so every time I had to name a variable it was a bit of a pickle :D
If you have feedback about style (or if there is a clang formatter I missed) it is welcome as well.

[saghul]

Ignore the style for now. It should just look like the surrounding code.

We have no linter alas.

[chqrlie]

In general, I'm supportive of the approach.

I didn't comment on the style issues since they are not relevant just yet ;-)

@chqrlie thoughts?

Hello @KaruroChori, sorry for the lag, I am focused on some tricky issues ATM, I won't have much time to investigate your approach until next week.

[KaruroChori]

No problem!

[KaruroChori]

I fixed this PR to not duplicate #424 & reworked some of the naming.