Split `fonts.rs` and `font.rs` into one file per type (#8500)

Pure move, no logic changes. First step of a chain that untangles
`Font`, `FontsImpl`, and `CachedFamily` ahead of font providers (#8491).

New files under `crates/epaint/src/text/`: `font_id.rs`, `font_data.rs`,
`font_tweak.rs`, `font_definitions.rs`, `glyph_rasterizer.rs`,
`galley_cache.rs`, `styled_metrics.rs`, `unicode.rs`.

* [x] I have followed the instructions in the PR template

PR chain, in order:
* #8490
* #8500
* #8501
* #8502
* #8503
* #8504
* #8491
* #8492
* #8493

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Co-authored-by: Claude Fable 5.1 <noreply@anthropic.com>