Add support for `prefer_precompressed` to FileServer

[3v0k4]

trafficstars

Fixes #5116

Todos:

• [ ] Update docs (eg, https://caddyserver.com/docs/caddyfile/directives/file_server#syntax)
• [ ] Review HTTP headers used when decompressing on-the-fly
• [ ] Tests
• [ ] Both compressed and precompressed used in the module, maybe only use precompressed?
• [ ] Need to handle URL canonicalization?
• [ ] TODO:s in code

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[3v0k4]

@francislavoie @mholt

This is still a spike as I'm trying to understand how we want to approach this before I do things properly.

Currently, I added a couple of if preferPrecompressed before the normal flow: the first one checks directly for the compressed file according to the client's accepted encodings and the second one looks for all the possible compressed files and, if one is found, uncompresses it.

I left the rest of the code because it takes care of the index files and browse. But we may want to consolidate things a bit more.

Would you be willing to share some feedback please?

[mholt]

Thanks for working on this -- I'll try to get around to this after the 2.8 release :)