Ability to symlink files to /usr/local/bin

[giggio]

Is your feature request related to a problem? Please describe.

I'd like to be able to simply add a script to /usr/local/bin/.

Describe the solution you'd like

Very similar to what is currently available to /etc:

      usr.local.bin = {
        foo = {
          text = ''
            This is just a test!
          '';
          target = "foo_test";
        };

Describe alternatives you've considered

Creating a full derivation to only symlink a single file to /run/system-manager/sw/bin/.

Additional context

I'm coming from home-manager, where this is easily doable for .local/bin with home.file.

This could be expanded to /usr/local, but that is not my immediate use case. Related: #37.