Add CI test verifying Mustache works with Deno

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.

.github/workflows/verify.yml

@@ -2,7 +2,7 @@ name: Verify changes
 
 on: [push, pull_request]
 
-jobs:       
+jobs:
   tests:
     runs-on: ubuntu-latest
 
@@ -71,3 +71,14 @@ jobs:
         run: |
           npm ci
           npx mocha test/module-systems/browser-test.js
+
+  deno-usage:
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v1
+      - uses: denolib/setup-deno@v1
+        with:
+          deno-version: 'v0.23.0'
+      - run: deno --version
+      - run: deno test --allow-net=deno.land test/module-systems/deno-test.ts

test/module-systems/deno-test.ts

@@ -0,0 +1,17 @@
+import { test } from "https://deno.land/std@v0.21.0/testing/mod.ts";
+import { assertEquals } from "https://deno.land/std@v0.21.0/testing/asserts.ts";
+import mustache from "../../mustache.mjs";
+
+const view = {
+  title: "Joe",
+  calc: function() {
+    return 2 + 4;
+  }
+};
+
+test(function canUseMustache() {
+  assertEquals(
+    mustache.render("{{title}} spends {{calc}}", view),
+    "Joe spends 6"
+  );
+});