Shaper abstraction setup (#38755)

# Objective

- Reorganise Servo's shaper code in preparation for multiple shaping
backends
- Make it possible to keep https://github.com/servo/servo/pull/38707
up-to-date with `main` with minimal conflicts

## Changes made

- Split `components/fonts/shaper.rs` into
`components/fonts/shapers/mod.rs` and
`components/fonts/shapers/harfbuzz.rs`
- Add traits for generic shapers
- `ShapedGlyphData` now takes ownership of the HarfBuzz buffer
(`hb_buffer_t`). This allows it to be returned from
the`THarfShaper::shape_text` function. The buffer is now deallocated in
the `ShapedGlyphData`s `Drop` impl.
- Add traits for HarfBuzz-like shapers and move code from
`save_glyph_results` function to be generic over those traits so that it
can be shared by a future `HarfRust` backend.

---------

Signed-off-by: Nico Burns <nico@nicoburns.com>

components/fonts/lib.rs

@@ -11,7 +11,7 @@ mod font_template;
 mod glyph;
 #[allow(unsafe_code)]
 pub mod platform;
-mod shaper;
+mod shapers;
 mod system_font_service;
 
 pub use font::*;
@@ -21,7 +21,7 @@ pub use font_template::*;
 pub use fonts_traits::*;
 pub use glyph::*;
 pub use platform::LocalFontIdentifier;
-pub use shaper::*;
+pub use shapers::*;
 pub use system_font_service::*;
 use unicode_properties::{EmojiStatus, UnicodeEmoji, emoji};