feat: the listing names the content types switched off

contract.ListingSettings.TypesOff lists the signpost keys of the content
types switched off in the sitemap settings, so another plugin (the SEO
tab's sitemap status) can tell an address left out with ReasonType
without reading the sitemap's tables. Additive.

contract/contract.go

@@ -95,7 +95,9 @@ type Listing interface {
 // path) and HTMLPageSize are the HTML sitemap's settings (the sitemap
 // serves its data at /_sitemap/api/v1/html); KeyFile is the IndexNow key
 // file name (<key>.txt, served at /_sitemap/indexnow/<key>.txt for the
-// site root), "" while IndexNow is off.
+// site root), "" while IndexNow is off. TypesOff lists the content types
+// (signpost keys) switched off in the sitemap settings, whose addresses
+// are left out with ReasonType.
 type ListingSettings struct {
 	SiteURL       string
 	IndexURL      string
@@ -105,6 +107,7 @@ type ListingSettings struct {
 	HTMLPaths     map[string]string
 	HTMLPageSize  int
 	KeyFile       string
+	TypesOff      []string
 }
 
 // GroupInfo is one file group: Key is the group (as Entries takes it), Slug