WIP Multiple font size / line height spans

[tigregalis]

Current status: Almost done. Working, with some fixes for the editing required.

Fixes #64

Implement specifying multiple font sizes and/or line heights.

https://github.com/pop-os/cosmic-text/assets/38416468/60009d8f-74cf-410f-9c12-9ccff4979110

TODO:

• [x] fix scrolling
• [x] implement enum LineHeight
• [x] cache line_heights()
• [x] retire Metrics
• [x] rebase
• [ ] migrate example editor-orbclient
  • [ ] fix bug empty buffer does not render cursor
  • [ ] fix bug empty buffer has default Attrs
  • [ ] support rescaling font sizes on demand, perhaps via mutable access to spans of Attrs
• [ ] migrate example editor-test
• [ ] migrate example editor-libcosmic (NOTE: does not compile on Windows with pinned version of iced (incompatible API))
• [ ] migrate example terminal (NOTE: does not compile on Windows with termion, see https://github.com/pop-os/cosmic-text/pull/129)
• [ ] migrate tests
• [ ] migrate benches
• [ ] migrate vi feature
• [ ] fix editor action Vertical
• [ ] fix some editor actions not redrawing
• [x] tidy up
• [ ] docs and PR write-up

Future work:

• Split Attrs: color_opt, metadata, font_size, line_height
• Buffer should have a default Attrs
• Convenience method for updating all spans of Attrs in a Buffer
• Handle start-of-line Attrs more carefully

Questions:

• Should the Hash impl of Attrs include line height?
• Should Metrics be restored as a property of Attrs, with definition Metrics { font_size: f32, line_height: LineHeight }?

[nicoburns]

Linking my PR that continues this PR: https://github.com/tigregalis/cosmic-text/pull/1