Reduce usage of SystemJS, in the development viewer, even further

With these changes SystemJS is now only used, during development, on the worker-thread and in the unit/font-tests, since Firefox is currently missing support for worker modules; please see https://bugzilla.mozilla.org/show_bug.cgi?id=1247687

Hence all the JavaScript files in the `web/` and `src/display/` folders are now loaded *natively* by the browser (during development) using standard `import` statements/calls, thanks to a nice `import-maps` polyfill.

*Please note:* As soon as https://bugzilla.mozilla.org/show_bug.cgi?id=1247687 is fixed in Firefox, we should be able to remove all traces of SystemJS and thus finally be able to use every possible modern JavaScript feature.

src/core/.eslintrc

@@ -0,0 +1,13 @@
+{
+  "parserOptions": {
+    "ecmaVersion": 2017,
+  },
+
+  "extends": [
+    "../../.eslintrc"
+  ],
+
+  "env": {
+    "es2017": true,
+  },
+}

src/display/api.js

@@ -1650,7 +1650,9 @@ const PDFWorker = (function PDFWorkerClosure() {
       }
       if (typeof PDFJSDev === "undefined" || !PDFJSDev.test("PRODUCTION")) {
         if (typeof SystemJS !== "object") {
-          throw new Error("SystemJS must be used to load fake worker.");
+          // Manually load SystemJS, since it's only necessary for fake workers.
+          await loadScript("../node_modules/systemjs/dist/system.js");
+          await loadScript("../systemjs.config.js");
         }
         const worker = await SystemJS.import("pdfjs/core/worker.js");
         return worker.WorkerMessageHandler;

src/pdf.js

@@ -65,8 +65,8 @@ const pdfjsBuild =
 
 if (typeof PDFJSDev === "undefined" || !PDFJSDev.test("PRODUCTION")) {
   const streamsPromise = Promise.all([
-    SystemJS.import("pdfjs/display/network.js"),
-    SystemJS.import("pdfjs/display/fetch_stream.js"),
+    import("pdfjs/display/network.js"),
+    import("pdfjs/display/fetch_stream.js"),
   ]);
   setPDFNetworkStreamFactory(params => {
     return streamsPromise.then(streams => {