[ty] Add separate type for typevar "identity" #55404

Job	Run time
cargo fmt	21s
cargo build (release)	0s
python package	1m 30s
pre-commit	1m 47s
mkdocs	2m 50s
Determine changes	13s
cargo test (linux)	3m 40s
cargo test (windows)	6m 59s
cargo clippy	1m 54s
cargo test (linux, release)	7m 8s
cargo test (wasm)	3m 24s
test scripts	1m 0s
cargo shear	25s
check playground	0s
benchmarks instrumented (ty)	10m 6s
benchmarks walltime (medium\|multithreaded)	8m 31s
benchmarks walltime (small\|large)	8m 3s
ty completion evaluation	1m 20s
cargo build (msrv)	1m 57s
cargo fuzz build	3m 56s
formatter instabilities and black similarity	0s
benchmarks instrumented (ruff)	0s
ecosystem	47s
test ruff-lsp	57s
Fuzz for new ty panics	16m 55s
fuzz parser	-1s
	1h 23m 42s

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