This adds both the recommended TypeScript checks, plus the recommended
TypeScript checks that require type checking. This latter addition
means that eslint essentially has to compile all of the TypeScript in
the project, causing it to be slower. This isn't much of a problem in
VS Code because there's a lot of caching being done, but it's clearly
slower when run on the commandline.
All of the errors are either fixed or suppressed. Some errors are
suppressed because fixing them would be too laborious for the little
value gained.
The eslint config is also slightly refactored to separate the strictly
TypeScript checks from the JavaScript checks.
* 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
