`bevy_text` migration from Cosmic Text to Parley

bevy_text now uses Parley for its text layout. For the most part, this change should be invisible to users of bevy_text and Bevy more broadly.

However, some low-level public methods and types (such as FontAtlasKey) have changed to map to parley's distinct API.

This migration should be relatively straightforward. Use the linked PR as an example of the correct migration, but please ask for help (and explain your use case) if you run into difficulties not noted below.

Known migration steps:

• System font discovery now requires you to enable the bevy/system_font_discovery feature. Users on Linux will need the fontconfig library for this. On Ubuntu, this can be done using sudo apt install libfontconfig1-dev.
• The various methods for setting the fallback font (such as set_serif_family, set_sans_serif_family or set_monospace_family) now return a Result. These will fail if the provided font is not found. By-and-large, you should not need to call these methods: font fallback is handled automatically via fontique through parley, using the system-provided fallback fonts (but see the above note about system font discovery).