Adding syntax-only and plain diagnostics output flags

[certik]

Original issue: https://gitlab.com/lfortran/lfortran/-/issues/708

If lfortran is to be used as a standalone linter (which I would very much like to!), like gfortran and ifort are in our VS Code Modern Fortran extension, I believe it would need to have a flag similar to gfortran's -fsyntax-only, where no actual compilation occurs and only the validity of the syntax is checked.

I would also consider adding a convenience routine for standardising the output of the diagnostics, similar to gfortran's -fdiagnostics-plain-output. (Note that -fdiagnostics-plain-output works on gfortran >= 11.0).

Let me know if that sounds reasonable and doable.

[gnikit]

This also addresses #331 so we can go ahead and close that issue.