Run linters in parallel

[Timmmm]

Since the filesystem is virtualised by WASI, we could support running linters in parallel. Each linter would see the files on disk, and we would intercept writes in memory. Once all have been run we can write them back to disk, using some ordering for files that have been written by multiple linters.

A tricky part is that you might want linters to run serially on modified files, e.g. when formatting Python it's common to run isort and then black.