Extract `LineIndex` independent methods from `Locator` #34572

Job	Run time
Determine changes	9s
cargo fmt	19s
mkdocs	3m 14s
pre-commit	41s
python package	1m 26s
cargo clippy	2m 21s
cargo build (msrv)	3m 55s
benchmarks	4m 49s
cargo build (release)	5m 20s
cargo fuzz	4m 41s
cargo test (linux)	5m 1s
cargo shear	28s
cargo test (wasm)	1m 51s
formatter instabilities and black similarity	4m 14s
cargo test (windows)	9m 35s
test scripts	1m 16s
test ruff-lsp	14s
Fuzz the parser	5m 20s
ecosystem	14m 29s
	1h 9m 23s

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