stubgen: Add support for PEP 570 positional-only parameters #12038

Job	Run time
Test mypyc suite with 32-bit Python	8m 25s
Test suite with py38-ubuntu, mypyc-compiled	18m 40s
Test suite with py38-windows-64	25m 57s
Test suite with py39-ubuntu	18m 44s
Test suite with py310-ubuntu	17m 39s
Test suite with py311-ubuntu, mypyc-compiled	17m 42s
Test suite with py312-ubuntu, mypyc-compiled	19m 55s
mypyc runtime tests with py39-macos	5m 23s
mypyc runtime tests with py38-debug-build-ubuntu	5m 12s
Type check our own code (py38-ubuntu)	1m 43s
Type check our own code (py38-windows-64)	2m 15s
Formatting and code style with Black + ruff	36s
	2h 22m 11s

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