Introduces a more robust keybinding system. Previously, keybinds were hardcoded into literal `if key = this and modifier = that`. Now, we have a system for defining keybindings in a human friendly way i.e. `super+c=copy_from_clipboard` and a system internally for looking up and executing keybind actions.
There's still a lot more keybinds we can add support for but now that this system is in place it should be easy to do as we get there. Namely, we need to fully implement this: https://invisible-island.net/xterm/ctlseqs/ctlseqs.html#h2-PC-Style-Function-Keys
You can now set custom keybinds by specifying the `--keybind` flag to the program. Example: `ghostty --keybind=super+c=copy_from_clipboard`. You can repeat this flag to specify more keybinds. The full set of actions available is in `src/input/Binding.zig` under `Action`.
Many changes to support emoji and wide chars:
- Freetype built with libpng support (some emoji font use pngs)
- Introduced `font.FallbackSet` structure for looking for glyphs in multiple fonts so we can find emojis
- Texture atlas supports 3 and 4 channel colors (RGB/RGBA, although emojis are in BGRA)
- `font.Face` supports loading colored fonts and glyphs
- `font.Face` supports fonts that do not support variable pixel sizes
- Shader supports two textures (greyscale and colored)
- Shader will downsample glyphs that don't fit into the cell size because some fixed size emoji fonts are large
- Shader supports wide (2x) cells across all types: cursor, fg, bg, underline
- Terminal state supports detecting and managing wide (2x) chars by looking them up in the Unicode EastAsianWidth database and populating "spacer" cells.
- Selection (and clipboard ops) support wide chars
