[api-minor] Only support browsers/environments that have *basic* support for Promise natively

Based on https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise#Browser_compatibility and https://caniuse.com/#feat=promises, all even remotely modern browsers already support *basic* `Promise` functionality natively.

The only reason for keeping the `Promise` polyfill (at all) is to be able to support recent additions to the specification, such as e.g. `finally` and `allSettled`.
Note that this patch will, on its own, remove support for IE 11/Edge (non-Chromium based) in both the general PDF.js library and the default viewer.

README.md

@@ -24,7 +24,7 @@ Feel free to stop by our [Matrix room](https://chat.mozilla.org/#/room/#pdfjs:mo
 ### Online demo
 
 Please note that the "Modern browsers" version assumes native support for
-features such as e.g. `async`/`await`, `Promise`, and `ReadableStream`.
+features such as e.g. `async`/`await`, and `ReadableStream`.
 
 + Modern browsers: https://mozilla.github.io/pdf.js/web/viewer.html
 

external/dist/README.md

@@ -8,7 +8,6 @@ This is a pre-built version of the PDF.js source code. It is automatically
 generated by the build scripts.
 
 For usage with older browsers/environments, without support for modern features
-such as e.g. `async`/`await`, `Promise`, and `ReadableStream`,
-please refer to the `es5` folder.
+such as e.g. `async`/`await`, and `ReadableStream`, please see the `es5` folder.
 
 See https://github.com/mozilla/pdf.js for learning and contributing.

src/shared/compatibility.js

@@ -242,14 +242,16 @@ if (
     require("core-js/es/typed-array/slice");
   })();
 
-  // Support: IE, Safari<11, Chrome<63
+  // Provides support for *recent* additions to the Promise specification,
+  // however basic Promise support is assumed to be available natively.
+  // Support: Firefox<71, Safari<13, Chrome<76
   (function checkPromise() {
     if (typeof PDFJSDev !== "undefined" && PDFJSDev.test("IMAGE_DECODERS")) {
       // The current image decoders are synchronous, hence `Promise` shouldn't
       // need to be polyfilled for the IMAGE_DECODERS build target.
       return;
     }
-    if (globalThis.Promise && globalThis.Promise.allSettled) {
+    if (globalThis.Promise.allSettled) {
       return;
     }
     globalThis.Promise = require("core-js/es/promise/index.js");