Add LFortran support

[wpbonelli]

Builds on #57, opening new PR as I don't have permission to push directly to your branch @gha3mi. Thanks for putting in the main effort here.

I think we can just use the builtin conda on linux/windows and install/use micromamba on mac, and move ahead with this, rather than wait for it on other package managers as I was thinking before. Some more things:

• this now installs lfortran to the base environment, not a custom one
• lfortran seems to pair better with MSVC on Windows — install MSVC toolchain with it and set cl as the C/C++ env vars and outputs, still use gcc/g++ on linux and mac though
• tweaked the testing conditions, there is some issue with lfortran executable discovery on powershell and command prompt, so windows support is limited to bash for now unless it's invoked by abs path
• updated the test matrix and compatibility files

[gha3mi]

Thanks, @wpbonelli, and sorry about that. I forgot to give permission, but it is possible now.

[wpbonelli]

thanks @gha3mi, I will close this