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/font.rs

@@ -52,6 +52,7 @@ pub const SBIX: OpenTypeTableTag = ot_tag!('s', 'b', 'i', 'x');
 pub const CBDT: OpenTypeTableTag = ot_tag!('C', 'B', 'D', 'T');
 pub const COLR: OpenTypeTableTag = ot_tag!('C', 'O', 'L', 'R');
 pub const BASE: OpenTypeTableTag = ot_tag!('B', 'A', 'S', 'E');
+pub const LIGA: OpenTypeTableTag = ot_tag!('l', 'i', 'g', 'a');
 
 pub const LAST_RESORT_GLYPH_ADVANCE: FractionalPixel = 10.0;