[api-minor] Move the ReadableStream polyfill to the global scope

Note that most (reasonably) modern browsers have supported this for a while now, see https://developer.mozilla.org/en-US/docs/Web/API/ReadableStream#Browser_compatibility

By moving the polyfill into `src/shared/compatibility.js` we can thus get rid of the need to manually export/import `ReadableStream` and simply use it directly instead.

The only change here which *could* possibly lead to a difference in behavior is in the `isFetchSupported` function. Previously we attempted to check for the existence of a global `ReadableStream` implementation, which could now pass (assuming obviously that the preceding checks also succeeded).
However I'm not sure if that's a problem, since the previous check only confirmed the existence of a native `ReadableStream` implementation and not that it actually worked correctly. Finally it *could* just as well have been a globally registered polyfill from an application embedding the PDF.js library.

src/display/display_utils.js

@@ -435,7 +435,6 @@ class StatTimer {
 function isFetchSupported() {
   return (typeof fetch !== 'undefined' &&
           typeof Response !== 'undefined' && 'body' in Response.prototype &&
-          // eslint-disable-next-line no-restricted-globals
           typeof ReadableStream !== 'undefined');
 }