Add Github Desktop integration

Add integration with Github Desktop

for the editor integration guidance ;

This is the checklist of things that it needs to support:

• the editor supports opening a directory, not just a file
• the editor is installed by the user, so there is a reliable way to find it on the user's machine
• it comes with a command-line interface that can be launched by Desktop

While Dr Racket supports the second and third options, it doesn't seem to support the first requirement;

Miriams-MBP:~ spdegabrielle$ drracket --help
DrRacket [ ... ] [] ...
where is one of
--help, -h : Show this help
-- : Do not treat any remaining argument as a switch (at this level)
Multiple single-letter switches can be combined after one -'; for example: -h-' is the same as `-h --'
Miriams-MBP:~ spdegabrielle$ drracket /Users/spdegabrielle/Dev/

... returns the error Cannot open /Users/spdegabrielle/Dev/ because it does not exist.

That said, I believe adding opening a directory might be a good first issue; it would require changing the command line options to detect when it is being passed a directory rather than a file, and triggering the File=>Open dialog for the named directory.

Integration also requires changes to GitHub desktop - but they already support a number of other editors across win/mac/linux and it seems well documented;

The source for the editor integration on Windows is found in
app/src/lib/editors/win32.ts.

The source for the editor integration on macOS is found in
app/src/lib/editors/darwin.ts.

The source for the editor integration on Linux is found in
app/src/lib/editors/linux.ts.

Guidance on editor integration is at: https://github.com/desktop/desktop/blob/development/docs/technical/editor-integration.md

Start by reproducing the directory argument behavior with the documented drracket --help command and review the editor integration guidance. The GitHub Desktop integration files mentioned are app/src/lib/editors/win32.ts, darwin.ts, and linux.ts, plus docs/technical/editor-integration.md; done means directory opening and the required integration support work across the relevant platforms.