Change `OnceCell` to `OnceLock` in `TabExpandedString` #995

Job	Run time
cross (armv5te-unknown-linux-gnueabi)	1m 22s
msrv	22s
audit	17s
lint	36s
test (ubuntu-latest, stable, --all-features)	24s
test (macos-latest, stable, --all-features)	29s
test (windows-latest, stable, --all-features)	1m 42s
test (ubuntu-latest, stable)	19s
test (ubuntu-latest, beta, --all-features)	27s
test (ubuntu-latest, stable, --all-features, --target armv5te-unknown-linux-gnueabi, true)	23s
	6m 21s

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