Add Brotli decompressor

[sorairolake]

Brotli is primarily used for HTTP compression. The standard library implements HTTP, and the deflate, gzip and zstd decompressors are already implemented. I think Brotli is widely used for modern HTTP compression. So I think it would be better to implement this data compression algorithm in the standard library.

[kcinnu]

im willing to code a decompressor if this gets accepted (no guarantees about whether i will be able to, though)