add import, export pane to local filesystem

[bourgeoa]

This can follow the W3C filesystem draft https://wicg.github.io/file-system-access/
This is used in some libraries like https://github.com/jimmywarting/native-file-system-adapter or https://github.com/GoogleChromeLabs/browser-fs-access/

A live demo of the later https://browser-fs-access.glitch.me/.

[josephguillaume]

Is this a pane to import/export files, or a feature to import/export panes? (and the latter therefore related to dynamic loading of panes SolidOS/pane-registry#28)

[bourgeoa]

The idea was to import/export files.