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.

testcases/t/201-config-parser.t

@@ -116,6 +116,7 @@ is(parser_calls($config),
 
 $config = <<'EOT';
 assign [class="^Chrome"] 4
+assign [class="^Chrome"] workspace number 3
 assign [class="^Chrome"] named workspace
 assign [class="^Chrome"] "quoted named workspace"
 assign [class="^Chrome"] → "quoted named workspace"
@@ -123,13 +124,15 @@ EOT
 
 $expected = <<'EOT';
 cfg_criteria_add(class, ^Chrome)
-cfg_assign(4)
+cfg_assign(4, 0)
 cfg_criteria_add(class, ^Chrome)
-cfg_assign(named workspace)
+cfg_assign(3, 1)
 cfg_criteria_add(class, ^Chrome)
-cfg_assign(quoted named workspace)
+cfg_assign(named workspace, 0)
 cfg_criteria_add(class, ^Chrome)
-cfg_assign(quoted named workspace)
+cfg_assign(quoted named workspace, 0)
+cfg_criteria_add(class, ^Chrome)
+cfg_assign(quoted named workspace, 0)
 EOT
 
 is(parser_calls($config),