[githubfs] Load file from raw.githubcontent.io instead of API

It may be possible to reduce the API usage, and therefore the possibility of hitting the API limit, by loading file from `https://raw.githubcontent.io/` whenever possible, for example, when a file is directly requested.

Comments? @Somae @onsetsu @JensLincke

No files or tests are named; start by locating the githubfs file-loading path in the service worker and tracing direct file requests through the GitHub API. Compare when raw.githubcontent.io can be used instead, and consider the work done when eligible requests avoid the API without breaking file access.

Written by the indexing model from the issue text.