Create a FileChooser component using showOpenFilePicker

[iflan]

Before, FileModal always used a file input control for selecting local files. This allowed the emulator to read from the file, but precluded writing back to the file.

With this change, the FileModal delegates to the new FileChooser component. The FileChooser will use showOpenFilePicker if it is available and a regular file input if it's not.

Using showOpenFilePicker has the advantage of allowing the emulator to write back to the file (if the user grants permission). While the emulator does not yet take advantage of this write capability, that will come.

[iflan]

I would just like to say that this is my first time trying to use React/Preact, so I may be doing everything wrong. Feedback is welcome!