add commonly used file extensions

[st-h]

from emberjs/rfcs#648 this PR adds default support for file suffixes: svg, webp, mp4, webmanifest and json

[sandstrom]

Good idea!

No biggie, but is 'webmanifest' common enough to warrant default inclusion?

I'd never heard of it and browser support isn't very good (not supported by Safari, for example). Still labeled as "experimental" by MDN.

[st-h]

@sandstrom sorry about not getting back to this any earlier. Support isn't that bad per se: https://caniuse.com/?search=manifest

Additionally it seems to be better to support more commonly used items than less, since this also configures which file types to use the prepend option for. Any missing file types will cause loading of these resources to fail.

Otherwise: is there any issue which is preventing us from merging? I had opened an RFC previously and there did not seem to be any objections.