Add support for time-synchronized subtitles via WebVTT

[TimJentzsch]

For cut-scenes and in-game dialogue, it would be very useful to provide support for time-synchronized text. This can serve as captions or subtitles.

The most suitable format to provide that feature seems to be WebVTT. It is used in the web stack for videos, but also adopted in other areas like YouTube. It allows you to specify timed cues, as well as positioning and styling for them.

Since styling is still being developed in bevy_ui and the limited support for font families, simple support for plain-text cues is sufficient for now. Of course, the user should be able to define the subtitles in multiple languages, so this feature should use the localized assets from #10 as well.