media: support application/manifest+json

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