feat: Add recursion_mode ('immediate' or 'deferred') to Signal and SignalInstance

[tlambert03]

fixes #292, and represents a partial reversion of #281 (the behavior of #281 is still possible, but must be opted into using recursion_mode = 'deferred')

This adds the possibility to control the order callbacks during recursive emission events (when one of the callbacks itself emits the same event):

• "immediate": Nested emission events immediately begin a deeper emission loop: calling all callbacks with the arguments of the nested emitter before returning to continue the original emission loop. This is the default behavior.
• "deferred": Nested emission events are deferred until the current emission loop is complete. This is the default behavior.

@Czaki, thoughts welcome on parameter naming and signature, etc... (i.e. should it be a boolean recurse_immediately instead of a set of literal strings? will we ever want additional modes?)

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 100.00%. Comparing base (c643ad0) to head (1589cec).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #293 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 22 22 Lines 1933 1973 +40 ========================================= + Hits 1933 1973 +40 ```

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[codspeed-hq[bot]]

CodSpeed Performance Report

Merging #293 will improve performances by 10.36%

_{Comparing tlambert03:recursion-mode (1589cec) with main (c643ad0)}

Summary

⚡ 1 improvements ✅ 65 untouched benchmarks

Benchmarks breakdown

	Benchmark	main	tlambert03:recursion-mode	Change
⚡	test_emit_time[setattr-50]	225.7 µs	204.5 µs	+10.36%

[tlambert03]

The failing benchmarks represent a slight increase in the time to create a signal instance:

Change	Before [f4ac7c3f] <v0.10.0^0>	After [b17be85c]	Ratio	Benchmark (Parameter)
+	477±3ns	833±3ns	1.74	benchmarks.CreateSuite.time_create_signal_instance
+	4.80±0.05μs	5.87±0.08μs	1.22	benchmarks.EventedSetSuite.time_create_set(10)
+	4.85±0.03μs	5.92±0.02μs	1.22	benchmarks.EventedSetWithCallbackSuite.time_create_set(10)

which is not something I'm worried about here given that it's on the order of ns.

@Czaki, this is ready for final review. I'll wait for your review on this one before merging. And I intend to follow up in another PR reviewing the exception handling logic slightly