io/fs: compose multiple FS into one

[icholy]

There's no way to combine multiple fs.FS values into a single tree. I wanted to create a zip archive that includes files from more than one fs.FS, but zip.Writer.AddFS only takes one fs.FS and doesn't allow specifying a prefix. fs.Sub lets you carve out part of a tree, but there’s no complementary way to merge multiple trees. This feels like a missing piece in io/fs.

An example of what an API could look like (inspired by https://github.com/lestrrat-go/multifs):

var vfs fs.Overlay

vfs.Mount("a", fs1)
vfs.Mount("b", fs2)

[seankhliao]

This should fall into the category of https://go.dev/doc/faq#x_in_std until there is strong evidence that it's a widely used function.