Support native filtering of |data:, |blob: [firefox-webext]

[gorhill]

In Firefox-webext, data: and blob: URIs can be seen by a webRequest.onBeforeRequest listener, despite the documentation implying this is not the case:

The special value "<all_urls>" matches all URLs under any of the supported schemes: that is, "http", "https", "file", "ftp", "app".

However when I use <all_urls>, I see data: URIs being passed on uBO's webRequest.onBeforeRequest listener. (Test URL: https://www.google.ca/search?q=images&tbm=isch, images in top rows are typically data: URI-based).

uBO converts a filter such as |data: into a csp=-based filter, since the webRequest.onBeforeRequest API of Chromium-based browsers can't see data: or blob: URIs.

Support native data:/blob: filtering when the webext API of a platform can support it, convert into a csp= filter only when the platform cannot support it.