mu: add 'scm' command

Add the "scm" command, the way to get a Guile/Scheme shell.

This is experimental but will replace the current guile support in guile/ at
some point.

meson_options.txt

@@ -35,7 +35,7 @@ option('lispdir',
 
 
 #
-# guile
+# guile & scm
 #
 
 option('guile',
@@ -48,6 +48,11 @@ option('guile-extension-dir',
        type: 'string',
        description: 'custom install path for the guile extension module')
 
+option('scm',
+       type : 'feature',
+       value: 'auto',
+       description: 'build the guile scripting support (requires guile-3.x)')
+
 #
 # misc
 #