[CHANGED] refactored nats.c, prep for js_PullSubscribeAsync

[levb]

This is a replacement for https://github.com/nats-io/nats.c/pull/772 that had too many changes/re-bases and had grown too complicated for review.

• Code organization - broke up nats.c into ./glib/*.c
• Added nats_OpenWithConfig for thread pool control, and in preparation for JSON-ifying default options, a separate request from @jnmoyne
• [EXPERIMENTAL, not used/tested] Allow using a thread pool for reply delivery.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 74.38380% with 291 lines in your changes missing coverage. Please review.

Project coverage is 68.80%. Comparing base (1e675d6) to head (50506e8).

Files	Patch %	Lines
src/glib/glib.c	69.53%	21 Missing and 57 partials :warning:
src/glib/glib_last_error.c	64.82%	18 Missing and 33 partials :warning:
src/nats.c	56.97%	28 Missing and 9 partials :warning:
src/sub.c	83.48%	12 Missing and 25 partials :warning:
src/glib/glib_dispatch_pool.c	64.93%	11 Missing and 16 partials :warning:
src/glib/glib_timer.c	81.50%	3 Missing and 24 partials :warning:
src/glib/glib_async_cb.c	82.81%	5 Missing and 6 partials :warning:
src/glib/glib_gc.c	78.57%	2 Missing and 4 partials :warning:
src/js.c	81.48%	1 Missing and 4 partials :warning:
src/timer.c	25.00%	1 Missing and 2 partials :warning:
... and 6 more

Additional details and impacted files

```diff @@ Coverage Diff @@ ## lev-bench2 #778 +/- ## ============================================== - Coverage 68.84% 68.80% -0.04% ============================================== Files 39 49 +10 Lines 15238 15206 -32 Branches 3153 3135 -18 ============================================== - Hits 10491 10463 -28 - Misses 1680 1701 +21 + Partials 3067 3042 -25 ```

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[levb]

performance benchmark: https://github.com/nats-io/nats.c/actions/runs/10267478369/job/28408116810?pr=778

Step: compare benchmarks

[levb]

@kozlovic Can I please get an LGTM on https://github.com/nats-io/nats.c/pull/777 (the base) so I can merge cleanly?