[api-minor] Re-factor the basic textLayer-functionality

This is very old code, and predates e.g. the introduction of JavaScript classes, which creates unnecessarily unwieldy code in the viewer.
By introducing a new `TextLayer` class in the API, similar to how e.g. the `AnnotationLayer` looks, we're able to keep most parameters on the class-instance itself. This removes the need to manually track them in the viewer, and simplifies the call-sites.

This also removes the `numTextDivs` parameter from the "textlayerrendered" event, since that's only added to support default-viewer functionality that no longer exists.

Finally we try, as far as possible, to polyfill the old `renderTextLayer` and `updateTextLayer` functions since they are exposed in the library API.
For *simple* invocations of `renderTextLayer` the behaviour should thus be the same, with only a warning printed in the console.

web/pdfjs.js

@@ -53,6 +53,7 @@ const {
   renderTextLayer,
   setLayerDimensions,
   shadow,
+  TextLayer,
   UnexpectedResponseException,
   updateTextLayer,
   Util,
@@ -101,6 +102,7 @@ export {
   renderTextLayer,
   setLayerDimensions,
   shadow,
+  TextLayer,
   UnexpectedResponseException,
   updateTextLayer,
   Util,