feat!: Have `CustomType`s reference their `Extension` definition #2766

Job	Run time
Check for changes	5s
Build benchmarks 🏋️	2m 10s
tests (Rust 1.75)	2m 26s
check	18s
tests (Rust beta)	2m 53s
tests (Rust nightly, coverage)	3m 19s
tests (Rust stable, all features)	2m 18s
tests (Rust stable, no features)	1m 35s
tests hugr-llvm	0s
Check standard extensions	4s
Required checks 🦀	1s
	15m 9s

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