add string.py with constants #7381

Job	Run time
LPython CI (3.10, ubuntu-latest)	8m 43s
Build LPython to WASM	5m 51s
Test CPython Interop (@pythoncall)	1m 31s
LPython CI (3.10, macos-latest)	10m 17s
Build Jupyter Kernel	3m 20s
LPython CI (3.10, windows-2019)	12m 39s
Run Integration tests with Python 3.9	2m 22s
Check Release build	2m 21s
Run Integration tests with Python 3.10	1m 30s
Check Debug build	2m 31s
Run SymPy tests	1m 59s
Run Integration tests with Python 3.11	1m 30s
Run Integration tests with Python 3.12	1m 30s
Test PIP Installable Packages	1m 14s
Upload Tarball	32s
	57m 50s

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