Expose loading system fonts

[Dimchikkk]

Objective

Expose cosmic-text functionality to load system fonts.

Solution

Add load_system_fonts to TextPlugin struct.

Testing

cargo r --example text --release by setting load_system_fonts to true emoji is rendered correctly:

https://github.com/bevyengine/bevy/assets/111751109/63b25a6d-944b-43f9-8dac-da6de3e08809

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Migration Guide

TextPlugin started to have a field, so in places where TextPlugin was used have to use bevy_text::TextPlugin::default() or create a struct with load_system_fonts field.