Allow assign to workspace by number

Makes "assign [<criteria>] workspace number <number>" work in the same
manner as "move to workspace number <number>" instead of assigning the
window to a workspace named "number <number>".

config.spec is modified to expect a 'number' string and an extra
argument is used in cfg_assign.

For workspaces that don't exist yet, workspace_get is used as a
fallback. This also allows the user to assign to "<number> <workspace>"
eg "2: work" and the full name will be used if workspace number 2
doesn't exist yet.

Fixes #2590.

include/data.h

@@ -546,7 +546,8 @@ struct Assignment {
         A_ANY = 0,
         A_COMMAND = (1 << 0),
         A_TO_WORKSPACE = (1 << 1),
-        A_NO_FOCUS = (1 << 2)
+        A_NO_FOCUS = (1 << 2),
+        A_TO_WORKSPACE_NUMBER = (1 << 3)
     } type;
 
     /** the criteria to check if a window matches */