Publish to NPM registry.

[pauldruce]

Summary of changes:

• I've added a github workflow which will publish this project as a package on npmjs and the github npm registry.
  • This is essentially the same process that bats-core is using to publish as a node package.
  • This requires a secret called NPM_TOKEN for the existing project to work for the npmjs registry.
  • Publishing to the GitHub npm registry should work without any extra secrets.
• Listed bats and bats-support as actual dependencies, as bats-assert isn't much use without them.
• A smoke test of the npm installation was added to show it works.

This PR addressing #70

[pauldruce]

From looking at the current npmjs project. I expect that @jasonkarns should take a look at this, as in order to successfully publish to npmjs, we need an access token for the project.