Update the main, and pdfjs-dist, README to also include recent ECMASc…

…ript features

This adds "optional chaining" and "nullish coalescing" to the list of features needed when using the *modern*, i.e. non-translated/non-polyfilled, build of the PDF.js library.

README.md

@@ -24,7 +24,8 @@ 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`, and `ReadableStream`.
+features such as e.g. `async`/`await`, `ReadableStream`, optional chaining, and
+nullish coalescing.
 
 + Modern browsers: https://mozilla.github.io/pdf.js/web/viewer.html
 

external/dist/README.md

@@ -7,7 +7,8 @@ parsing and rendering PDFs.
 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`, and `ReadableStream`, please see the `es5` folder.
+For usage with older browsers or environments, without support for modern
+features such as e.g. `async`/`await`, `ReadableStream`, optional chaining, and
+nullish coalescing; please see the `es5` folder.
 
 See https://github.com/mozilla/pdf.js for learning and contributing.