add support for .webmanifest for favicon

[Buliwyfa]

Add support for files .webmanifest especially to support site.webmanifest use to define favicon, tab icon, app icon, etc.

It use on html head like this:

<link rel="manifest" href="site.webmanifest">

Suggested web.config entry is:

<mimeMap fileExtension=".webmanifest" mimeType="application/manifest+json" />

SOURCE: https://stackoverflow.com/questions/49566446/how-can-i-have-iis-properly-serve-webmanifest-files-on-my-web-site