Add support for x-enum-varnames to string enums #3140

Job	Run time
test_min_deps (macos-latest)	1m 5s
test_min_deps (windows-latest)	1m 38s
Integration Tests (pdm.lock)	22s
test_min_deps (ubuntu-latest)	37s
Integration Tests (pdm.minimal.lock)	24s
test (3.10, windows-latest)	57s
test (3.11, windows-latest)	57s
test (3.10, macos-latest)	1m 4s
test (3.10, ubuntu-latest)	1m 2s
test (3.11, macos-latest)	53s
test (3.12, macos-latest)	56s
test (3.11, ubuntu-latest)	1m 3s
test (3.13, macos-latest)	33s
test (3.12, windows-latest)	59s
test (3.12, ubuntu-latest)	59s
test (3.14, macos-latest)	50s
test (3.13, windows-latest)	1m 1s
test (3.13, ubuntu-latest)	1m 6s
test (3.14, windows-latest)	54s
test (3.14, ubuntu-latest)	1m 5s
Combine & check coverage	0s
	18m 25s

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