Fix error in bevy_ui when building without bevy_text

[BD103]

Objective

• bevy_ui does not build without the bevy_text feature due to improper feature gating.
  • Specifically, MeasureArgs<'a> had an unused lifetime 'a without bevy_text enabled. This is because it stores a reference to a cosmic_text::FontSystem.
  • This was caught by flag-frenzy in this run.

Solution

• Add a PhantomData to MeasureArgs<'a> in order to maintain its lifetime argument.
  • I also named it font_system, after the feature-gated argument that actually needs a lifetime, for usability. Please comment if you have a better solution!
• Move some unused imports to be behind the bevy_text feature gate.

Testing

# Fails on main.
cargo check -p bevy_ui --no-default-features
# Succeeds on main.
cargo check -p bevy_ui --no-default-features -F bevy_text

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Migration Guide

This is not a breaking change for users migrating from 0.14, since MeasureArgs did not exist then.

When the bevy_text feature is disabled for bevy_ui, the type of the MeasureArgs::font_system field is now a PhantomData instead of being removed entirely. This is in order to keep the lifetime parameter, even though it is unused without text being enabled.