[api-minor] Use the NodeCanvasFactory/NodeCMapReaderFactory classes as defaults in Node.js environments (issue 11900)

This moves, and slightly simplifies, code that's currently residing in the unit-test utils into the actual library, such that it's bundled with `GENERIC`-builds and used in e.g. the API-code.

As an added bonus, this also brings out-of-the-box support for CMaps in e.g. the Node.js examples.

gulpfile.js

@@ -1617,6 +1617,7 @@ gulp.task(
         bugs: DIST_BUGS_URL,
         license: DIST_LICENSE,
         browser: {
+          canvas: false,
           fs: false,
           http: false,
           https: false,