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.

docs/userguide

@@ -766,7 +766,7 @@ considered.
 
 *Syntax*:
 ------------------------------------------------------------
-assign <criteria> [→] [workspace] <workspace>
+assign <criteria> [→] [workspace] [number] <workspace>
 ------------------------------------------------------------
 
 *Examples*:
@@ -783,6 +783,12 @@ assign [class="^URxvt$"] → 2
 # Assignment to a named workspace
 assign [class="^URxvt$"] → work
 
+# Assign to the workspace with number 2, regardless of name
+assign [class="^URxvt$"] → number 2
+
+# You can also specify a number + name. If the workspace with number 2 exists, assign will skip the text part.
+assign [class="^URxvt$"] → number "2: work"
+
 # Start urxvt -name irssi
 assign [class="^URxvt$" instance="^irssi$"] → 3
 ----------------------