Use char index rather than position for indent slice #28946

Job	Run time
Determine changes	8s
mkdocs	2m 28s
cargo fmt	14s
pre-commit	31s
python package	1m 11s
cargo test (windows)	6m 54s
cargo build (release)	5m 43s
cargo clippy	1m 37s
cargo test (linux)	2m 59s
cargo test (wasm)	1m 12s
benchmarks	3m 54s
cargo fuzz	3m 54s
cargo shear	27s
test scripts	50s
formatter instabilities and black similarity	0s
ecosystem	9m 25s
test ruff-lsp	15s
Fuzz the parser	0s
	41m 42s

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