Fix failing tests on Windows with Go 1.20

all: Run gofumpt -l -w .

Fix concurrency issue in MemMapFs.Mkdir/MkdirAll

* The backing map is protected by a RWMutex
* This commit double checks for the existence of the directory inside the write lock to avoid potential data races when multiple goroutines tries to create
the same directory.

Fixes #361
Fixes #298

Make mem.File implement fs.ReadDirFile

Fix sorting in IOFS.ReadDir

We recently added a check for fs.ReadDirFile in IOFS.ReadDir, but forgot to apply a sort to the
result as defined in the spec.

This fixes that and adds a test case for it.

Update README.md

go mod tidy

Merge pull request #334 from KastenMike/add-modTime-to-folders

add modTime on folder creation

Merge pull request #332 from pojntfx/master

Call `OpenFile` instead of `Open` in `CacheOnReadFs` (fixes #193)

Merge pull request #333 from drakkan/gcsfs

GCS fs: move all gcsfs related implementations to its own package