Add support for racket packages (#27564)

- Add variants for various common build flags, including support for both versions of the Racket VM environment.
- Prevent `-j` flags to `make`, which has been known to cause problems with Racket builds.
- Prefer the minimal release to improve install times. Bells and whistles carry their own runtime dependencies and should be installed via `raco`. An enterprising user may even create a `RacketPackage` class to make spack aware of `raco` installed packages.
- Match the official version numbering scheme.

lib/spack/docs/build_systems.rst

@@ -51,6 +51,7 @@ on these ideas for each distinct build system that Spack supports:
    build_systems/perlpackage
    build_systems/pythonpackage
    build_systems/rpackage
+   build_systems/racketpackage
    build_systems/rubypackage
 
 .. toctree::