Allow assign to workspace by number

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.