enh: add LFortran as submodule with sparse checkout for `src/libasr` #7533

Job	Run time
LPython CI (3.10, macos-latest)	8m 58s
LPython CI (3.10, ubuntu-latest)	9m 22s
Test PIP Installable Packages	3m 31s
Build LPython to WASM	6m 9s
LPython CI (3.10, windows-2019)	12m 48s
Test LLVM 15	4m 45s
Check Release build	7m 11s
Run Integration tests with Python 3.10	4m 11s
Run Integration tests with Python 3.12	4m 3s
Test LLVM 16	4m 45s
Run SymPy tests	4m 27s
Test CPython Interop (@pythoncall)	3m 43s
Run Integration tests with Python 3.9	4m 3s
Test LLVM 10	4m 36s
Run Integration tests with Python 3.11	4m 8s
Build Jupyter Kernel	3m 30s
Upload Tarball	44s
Check Debug build	4m 40s
	1h 35m 34s

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