Implement string contains for lpython #7377

Job	Run time
Build Jupyter Kernel	3m 24s
LPython CI (3.10, ubuntu-latest)	7m 56s
Build LPython to WASM	5m 57s
LPython CI (3.10, macos-latest)	5m 45s
Test CPython Interop (@pythoncall)	1m 34s
LPython CI (3.10, windows-2019)	12m 45s
Check Debug build	1m 58s
Run Integration tests with Python 3.9	1m 57s
Run Integration tests with Python 3.10	1m 51s
Check Release build	1m 52s
Run Integration tests with Python 3.11	1m 48s
Run SymPy tests	2m 10s
Test PIP Installable Packages	1m 25s
Run Integration tests with Python 3.12	1m 46s
Upload Tarball	39s
	52m 47s

Provide feedback