Our test suite already has support for running mustache specs,
but looks like we've forgotten about them in CI for many years now.

Because it seems the standard library for v0.21 that was used in the
usage tests before has been removed, it returns 404 now..

With the introduction of a build step, where we take the source in .mjs
and build it into .js for non-ES Module systems, it's very important
that we remember to keep those two in sync.

That's what the CI job created in these changes ensure;

1. Run the build script
2. See if there are any pending git changes as a result

As long as there are no pending git changes, we're all fine and dandy,
or else we need to run the `npm run build` and commit the changes.

To ensure the ES module and source exposed by this package is compatible
with Deno going forward.

Added benefit is we get some sanity checks of the source code for free
due to Deno's built-in TypeScript compiler getting angry if it sees
things that does not make sense, in terms of missing function arguments
and so on.

This is a precursor to introducing a build step that will change what
we expose from this package. Better off writing some tests to verify
existing projects with different module systems continue to work as
expected.

This is a precursor to introducing a build step that will change what
we expose from this package. Better off writing some tests to verify
existing projects with different module systems continue to work as
expected.

Primarily because it's getting to be a pain in the backside to always
consider pre Node.js 6 when wanting to use up-to-date dependencies.

When initially creating the GitHub Actions
workflow, it was obviously forgotten to make
it also run on pull requests, not only when
changes are pushed to branches in the
original repository (not a fork).