filepicker: add support for abstracted filesystem io/fs.FS

[dolmen]

Add an optional io/fs.FS to filepicker's model to allow to browse an abstracted filesystem as an alternative to the full filesystem exposed by the os package.

The filepicker API is extended (and generalized thanks to io/fs.FS), without any breaking change.

This allows for example to browse limited a subdirectory by using os.Root (go1.24+):

fp := filepicker.New()
root, _ := os.OpenRoot(dir)
fp.FS = root.FS()

I have already submitted an implementation in #759.

[dolmen]

#954 is my implementation updated for the v2 API.