Add yarn dependabot checks

[NicMcPhee]

This adds Dependabot checks for the yarn (i.e., Node) dependencies for the project. Since Propeller can (in principle) run in ClojureScript, there are JavaScript dependencies that probably need to be updated semi-regularly for security reasons.

This adds an automated weekly check for out of date JavaScript dependencies, and will automatically create pull requests where there are updates that Dependabot thinks can be handled automatically.

We should at a minimum verify that the tests are passing before merging in a Dependabot PR, and given the incompleteness of the tests it would probably be good for someone to run a problem or two (in ClojureScript) as a sanity check to make sure things still work.

Actually, I'm not sure that there are any ClojureScript tests, so TBH I'm not sure what the best strategy is for determining if the ClojureScript version of the project is broken.