Add support for x-enum-varnames to string enums #3269

Job	Run time
test_min_deps (windows-latest)	1m 8s
test_min_deps (macos-latest)	42s
test_min_deps (ubuntu-latest)	43s
Integration Tests (pdm.lock)	28s
Integration Tests (pdm.minimal.lock)	27s
test (3.10, windows-latest)	1m 42s
test (3.10, macos-latest)	53s
test (3.11, windows-latest)	2m 5s
test (3.12, windows-latest)	1m 28s
test (3.10, ubuntu-latest)	1m 7s
test (3.14, ubuntu-latest)	56s
test (3.11, macos-latest)	41s
test (3.13, ubuntu-latest)	1m 11s
test (3.12, macos-latest)	47s
test (3.11, ubuntu-latest)	59s
test (3.13, macos-latest)	40s
test (3.14, windows-latest)	1m 32s
test (3.14, macos-latest)	43s
test (3.13, windows-latest)	1m 24s
test (3.12, ubuntu-latest)	1m 0s
Combine & check coverage	0s
	20m 36s

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