Bugfix in shorthand assignment operators

Shorthand assignment operators would throw since they were not updated to the new style of filter parsing.
Bugfix in assignments to a falsey value

Fixed a bug where an assignment that resulted in a falsey value would resolve into `null`.
Untrack project file
Added tests for shorthand assignment
Moved shell scripts to `bin`, headless tests, node/npm support

- Moved docs/gen to bin/docgen.
- Added bin/test for headless quint testing via node.
- Added package.json for node support and future npm support.