Add support for PIXI.BitmapText

[reececomo]

Example implementation of #393

All you need to do is override createTextField(...) to return BitmapText if that's what your app needs. You'll need to load or generate BitmapFonts yourself. See #393 thread for more examples.

import TaggedText from 'pixi-tagged-text';

export class MyText extends TaggedText {

  protected createTextField(token: TextSegmentToken, text: string, style: Partial<ITextStyle>): Text | BitmapText {
    // You can replace this logic with whatever you need in order to
    // load and style your BitmapFonts (e.g. resize or tint).
    return new BitmapText(text, { fontName: 'MyCustomFont' });
  }
}

[mimshwright]

Thanks very much for your PR. I appreciate the effort.
As noted in #393 there are some bigger issues with bitmapText and I'd rather not include a patch like this without more careful consideration.
For now, i'm afraid I'm going to reject this change to the library. But hopefully, your fork will be adequate for your needs.