* Create a FileChooser component using showOpenFilePicker
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.
* Addressed comments
* useState() instead of direct DOM manipulation
* backed out eslint changes in favor of suppressing the warning
