add a github action to test on push

[alisonatwork]

This PR introduces Github actions as CI so that we can get a full lint and test run for PRs and on master.

Some of the tests were a little flakey, so there are some small changes here:

1. Bring back the retries argument for the tests
2. Upgrade Mocha, since retries actually didn't work properly until 7.1.0
3. Use " instead of ' so that Prettier will run on Windows
4. Change the , and ; tests so that they don't reach into the extension to clear the last motion, just do a non-matching f first

The fourth one is necessary because when you run all of the test together, motions from previous tests will affect the outcome of those two tests. The previous behavior which did a reach around into ../../src doesn't work any more because with Webpack (#270) the actual running source has moved elsewhere.

I guess we could export the clear motion command as part of extension public API, but I think it would be nicer if the BlackBox tests didn't have to reach into the extension so much. There is another solution for this, and it is similar to something I am playing with in #258: we need to flag certain tests as requiring a "clean" document, and then those tests should get their own fresh tab. I think that is out of scope for this PR, though.

[alisonatwork]

Heads up: I merged this PR directly, because while it was on a fork the build would not be triggered anyway.

Here is the successful build on master: https://github.com/aioutecism/amVim-for-VSCode/actions/runs/127643761

Cc @aioutecism @jackfranklin @karlhorky and @tejanium for visibility.

[alisonatwork]

I was hoping after the merge to master, it would show up the build status of forks inside the PR, but I just rebase/force-pushed my branch over in #258 and no luck. I will look into this a bit more later. At least developers can check if the build worked in their own fork now.