Comply with documentation: 'workspace' token in 'assign' command

According to the User's Guide [1], an `assign` command allows a
`workspace` token after the selector, as an alternative or in addition
to the unicode arrow `→`. In reality, however, the `workspace` token is
not recognized.

Example:
  assign [class="Firefox"] workspace "1: Browser"
should assign Firefox windows to workspace `1: Browser`, but the the
browser window appears on a new workspace called
`workspace "1: Browser"` instead.

With this fix, both `→` and `workspace` are recognized (and ignored)
after the selector.

[1] https://i3wm.org/docs/userguide.html#_automatically_putting_clients_on_specific_workspaces

parser-specs/config.spec

@@ -146,6 +146,8 @@ state ASSIGN:
 state ASSIGN_WORKSPACE:
   '→'
       ->
+  'workspace'
+      ->
   workspace = string
       -> call cfg_assign($workspace)