Rename manifest.json to manifest.webmanifest

[andy128k]

It seems .webmanifest is a preferable extension these days.
It was reported in #280 and jantimon/favicons-webpack-plugin#272

[alexander-akait]

NOTE: File extension: .webmanifest or .json?
The IANA registered file extension for the manifest is .webmanifest. Some web servers recognize this extension and transfer the file using the standardized application manifest media type (application/manifest+json). Developers can also choose a different extension (e.g. .json) or none at all (e.g. /api/GetManifest), but are encouraged to transfer the manifest using the application/manifest+json MIME type, although any JSON MIME type is ok.

https://w3c.github.io/manifest/

Looks any extension is valid, but I am fine with this change

[andy128k]

While any extension is acceptable, it seems .webmanifest has some pros. OTOH it is still possible to change the name by passing a parameter manifestFileName.

[alexander-akait]

Yep, feel free to merge