Introduce action to lint new pull requests

[ryanwelcher]

This PR is a first step towards updating the CI processes on this repo. It does the following:

1. Updates learna.json to include all the examples
2. Changes the existing action to be run on pull request and run linting commands for JS, CSS and package.json files
3. Updates the code base where required to pass linting.

[gziolo]

Overall, I love the direction. Does it mean we won't run checks on commits to the main branch anymore?

[ryanwelcher]

Does it mean we won't run checks on commits to the main branch anymore?

I'm open to discussing this for sure. My feeling is that if everything has to be an approved PR before it gets to trunk we may not need it, but I guess it also can't hurt. Does anyone have access to push directly to trunk?

Is there a case I'm not considering?

[gziolo]

Is there a case I'm not considering?

There is one that is nearly impossible to happen here because of a low frequency of commits. In Gutenberg, we had many situations where you would open PR with all check green, then before merging it some other commits would introduce changes that combined with changes in the branch would cause issues. However, you won't catch those cases until you rebase the branch before merging or it will pop up once you merge PR into the main branch.