Replace the `BaseFontLoader` classes with one `FontLoader` class
Replace the `BaseFontLoader` classes with one `FontLoader` class

By having just *one* class, and using pre-processor blocks directly in the relevant methods, we reduce the size of this code in the *built* `pdf.js` file.
Originally, when the `BaseFontLoader` abstraction was added in PR 9982, the idea was probably that additional build-targets would get their own implementations. Given that this hasn't happened in the four years since that landed, it doesn't seem meaningful to keep it around.

Shoobx/pdf.js,Snuffleupagus/pdf.js,mozilla/pdf.js,Snuffleupagus/pdf.js,timvandermeij/pdf.js,Shoobx/pdf.js,showpad/mozilla-pdf.js,mozilla/pdf.js,Shoobx/pdf.js,showpad/mozilla-pdf.js,timvandermeij/pdf.js,mozilla/pdf.js,showpad/mozilla-pdf.js,Snuffleupagus/pdf.js,timvandermeij/pdf.js