Add support for x-enum-varnames to string enums #3270

Job	Run time
test_min_deps (windows-latest)	1m 51s
Integration Tests (pdm.lock)	30s
test (3.11, windows-latest)	1m 45s
test (3.10, macos-latest)	1m 3s
test_min_deps (macos-latest)	53s
test_min_deps (ubuntu-latest)	38s
test (3.13, macos-latest)	39s
test (3.13, ubuntu-latest)	1m 14s
test (3.14, macos-latest)	45s
test (3.12, windows-latest)	2m 3s
test (3.11, ubuntu-latest)	59s
test (3.11, macos-latest)	52s
test (3.14, ubuntu-latest)	1m 1s
test (3.10, ubuntu-latest)	1m 0s
test (3.12, macos-latest)	40s
Integration Tests (pdm.minimal.lock)	20s
test (3.10, windows-latest)	2m 7s
test (3.13, windows-latest)	1m 31s
test (3.12, ubuntu-latest)	1m 11s
test (3.14, windows-latest)	2m 3s
Combine & check coverage	16s
	23m 21s

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