As seen from the test run log:

```
$ deno test --allow-net=deno.land test/module-systems/deno-test.ts

Compile file:///home/runner/work/mustache.js/mustache.js/.deno.test.ts
Download deno.land/std@v0.51.0/testing/asserts.ts
Warning std versions prefixed with 'v' were deprecated recently. Please change your import to deno.land/std@0.51.0/testing/asserts.ts (at deno.land/std@v0.51.0/testing/asserts.ts)
error: Uncaught Error: Import 'deno.land/std@v0.51.0/testing/asserts.ts' failed: 404 Not Found
    at unwrapResponse ($deno$/ops/dispatch_json.ts:43:11)
    at Object.sendAsync ($deno$/ops/dispatch_json.ts:98:10)
    at async processImports ($deno$/compiler.ts:736:23)
```

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

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.