Restructure decompression code to allow for other algorithms

[wh0]

Currently we go through like this:

1. Refer to this thing we download as the .xz file
2. Check that the compression algorithm is actually "xz"
3. If not, raise an error
4. Decompress with xz

See the code: https://github.com/wh0/nix-cache-view/blob/1a5bba2d55272506ff2f7c6af27cf0bf92d79474/misc.js#L198

Change it to be more like:

1. Refer to this thing we download as the compressed file
2. Look at the compression algorithm
3. If it's "xz," decompress with xz
4. Otherwise, raise an error

At this time let's not actually add any other algorithms. But we should make it simple for when we do. Or maybe we could trivially support none?

[wh0]

680ce9ac32c5c19c3266874191bde963045a79c3