feat: make Signal and SignalInstance Generic, support static type validation of signal connections

[tlambert03]

supports the ability to perform static type checking on signal connection signatures

class Thing:
    sig = Signal(int, str)

t = Thing()

@t.sig.connect
def cb1() -> None: ...  # OK
@t.sig.connect
def cb2(a0: int) -> None: ...  # OK
@t.sig.connect
def cb3(a0: int, a1: str) -> None: ...  # OK

@t.sig.connect
def cb5(a0: int, a1: str, a2: float) -> None: ...  # ERR: too many params
@t.sig.connect
def cb6(a0: str) -> None: ...  # ERR: wrong type for a0

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 96.77419% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 99.95%. Comparing base (c4c83b2) to head (66a86ce).

Files	Patch %	Lines
src/psygnal/_signal.py	96.15%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #304 +/- ## =========================================== - Coverage 100.00% 99.95% -0.05% =========================================== Files 22 22 Lines 2071 2084 +13 =========================================== + Hits 2071 2083 +12 - Misses 0 1 +1 ```

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[codspeed-hq[bot]]

CodSpeed Performance Report

Merging #304 will degrade performances by 66.04%

_{Comparing tlambert03:generic-signal (66a86ce) with main (c4c83b2)}

Summary

⚡ 1 improvements ❌ 5 regressions ✅ 60 untouched benchmarks

:warning: Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Benchmark	main	tlambert03:generic-signal	Change
⚡	test_create_signal	144.7 µs	51.3 µs	×2.8
❌	test_create_signal_instance	51.9 µs	152.8 µs	-66.04%
❌	test_dataclass_group_create[attrs]	2.4 ms	3.1 ms	-22.91%
❌	test_dataclass_group_create[dataclass]	2 ms	2.7 ms	-24.91%
❌	test_dataclass_group_create[msgspec]	2.4 ms	3.1 ms	-22.15%
❌	test_dataclass_group_create[pydantic]	2.4 ms	3.1 ms	-22.39%