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]

Codecov Report

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

Project coverage is 99.95%. Comparing base (019232e) to head (cdb2ec3).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/psygnal/_signal.py	96.15%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##              main     #304      +/-   ##
===========================================
- Coverage   100.00%   99.95%   -0.05%     
===========================================
  Files           21       21              
  Lines         2062     2089      +27     
===========================================
+ Hits          2062     2088      +26     
- Misses           0        1       +1     

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[codspeed-hq]

CodSpeed Performance Report

Merging #304 will degrade performances by 56.98%

_{Comparing tlambert03:generic-signal (cdb2ec3) with main (019232e)}

Summary

⚡ 2 improvements
❌ 5 regressions
✅ 59 untouched benchmarks

🆕 1 new benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Benchmark	main	tlambert03:generic-signal	Change
⚡	test_create_signal	68.8 µs	28.2 µs	×2.4
❌	test_create_signal_instance	33.4 µs	77.6 µs	-56.98%
❌	test_dataclass_group_create[attrs]	536.8 µs	676.7 µs	-20.68%
❌	test_dataclass_group_create[dataclass]	523.1 µs	657.7 µs	-20.47%
❌	test_dataclass_group_create[msgspec]	544.9 µs	676 µs	-19.39%
❌	test_dataclass_group_create[pydantic]	541.2 µs	677.9 µs	-20.17%
🆕	test_emit_fast	N/A	21.7 µs	N/A
⚡	test_emit_time[partial-2]	69 µs	61.6 µs	+12.04%