Some hasJSActions, and general annotation-code, related cleanup in the viewer and API

- Add support for logical assignment operators, i.e. `&&=`, `||=`, and `??=`, with a Babel-plugin. Given that these required incrementing the ECMAScript version in the ESLint and Acorn configurations, and that platform/browser support is still fairly limited, always transpiling them seems appropriate for now.

 - Cache the `hasJSActions` promise in the API, similar to the existing `getAnnotations` caching. With this implemented, the lookup should now be cheap enough that it can be called unconditionally in the viewer.

 - Slightly improve cleanup of resources when destroying the `WorkerTransport`.

 - Remove the `annotationStorage`-property from the `PDFPageView` constructor, since it's not necessary and also brings it more inline with the `BaseViewer`.

 - Update the `BaseViewer.createAnnotationLayerBuilder` method to actaually agree with the `IPDFAnnotationLayerFactory` interface.[1]

 - Slightly tweak a couple of JSDoc comments.

---
[1] We probably ought to re-factor both the `IPDFTextLayerFactory` and `IPDFAnnotationLayerFactory` interfaces to take parameter objects instead, since especially the `IPDFAnnotationLayerFactory` one is becoming quite unwieldy. Given that that would likely be a breaking change for any custom viewer-components implementation, this probably requires careful deprecation.

external/builder/preprocessor2.js

@@ -8,7 +8,7 @@ var path = require("path");
 
 var PDFJS_PREPROCESSOR_NAME = "PDFJSDev";
 var ROOT_PREFIX = "$ROOT/";
-const ACORN_ECMA_VERSION = 2020;
+const ACORN_ECMA_VERSION = 2021;
 
 function isLiteral(obj, value) {
   return obj.type === "Literal" && obj.value === value;