media: support application/manifest+json

The standard file extension for Web App Manifest files is
".webmanifest". This commit allows Hugo to recognize .webmanifest files
as "application/manifest+json" files and to minify them using its
JSON minifier.

The .webmanifest file extension is recommended in the w3c spec to
simplify media type registration:
https://www.w3.org/TR/appmanifest/#media-type-registration

Webhint docs are also relevant:
https://webhint.io/docs/user-guide/hints/hint-manifest-file-extension/

Closes #8624

output/outputFormat.go

@@ -143,6 +143,14 @@ var (
 		Rel:         "alternate",
 	}
 
+	WebAppManifestFormat = Format{
+		Name:        "WebAppManifest",
+		MediaType:   media.WebAppManifestType,
+		BaseName:    "manifest",
+		IsPlainText: true,
+		Rel:         "manifest",
+	}
+
 	RobotsTxtFormat = Format{
 		Name:        "ROBOTS",
 		MediaType:   media.TextType,
@@ -176,6 +184,7 @@ var DefaultFormats = Formats{
 	CSVFormat,
 	HTMLFormat,
 	JSONFormat,
+	WebAppManifestFormat,
 	RobotsTxtFormat,
 	RSSFormat,
 	SitemapFormat,