feat: add regexp_instr function #40

Job	Run time
build / lint-python	10s
build / lint-rust	22s
build / generate-license	53s
build / lint-toml	6s
build / ManyLinux x86_64	1m 55s
build / macOS arm64 & Windows (3.10, windows-latest)	1m 59s
build / ManyLinux arm64	1m 52s
build / macOS arm64 & Windows (3.10, macos-latest)	2m 8s
build / build-macos-x86_64	0s
build / Source distribution	0s
build / Build docs	1s
build / Merge build artifacts	1s
test	0s
	9m 27s

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