Implement string contains for lpython (#2787) #7383

Job	Run time
Check Debug build	2m 52s
Build LPython to WASM	5m 43s
Build Jupyter Kernel	3m 26s
Test CPython Interop (@pythoncall)	1m 52s
LPython CI (3.10, ubuntu-latest)	8m 42s
LPython CI (3.10, macos-latest)	10m 43s
Run Integration tests with Python 3.9	2m 7s
LPython CI (3.10, windows-2019)	12m 31s
Check Release build	3m 16s
Run SymPy tests	2m 25s
Run Integration tests with Python 3.10	2m 1s
Test PIP Installable Packages	1m 33s
Upload Tarball	36s
Run Integration tests with Python 3.11	1m 55s
Run Integration tests with Python 3.12	2m 0s
	1h 1m 42s

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