feat(wasm): port Ghostty's wasm32 build

Ports main_wasm.zig as crates/ghostty-wasm (cdylib exporting the Zig
names: atlas_*, face_*, deferred_face_*, shaper_*, config_*), the
web_canvas font face and shaper (backend selected on wasm32), os/wasm
(logging, target, JS interop) and config/Wasm.zig.

C-library dependencies (freetype, harfbuzz, fontconfig, onig, gettext,
D-Bus, rustix) are target-gated. Like Zig's wasm build, link regex
search (StringMap::search_iterator) is unavailable without Oniguruma;
Collection/SharedGrid/harfbuzz are compiled out on wasm32. ghostty-os
switches from forbid to deny unsafe_code for the JS import blocks.

The JS import ABI is currently a documented handle-based interface with
a reference host (js/ghostty_host.js); aligning it with zig-js's exact
import names follows separately.

Test Plan:
- cargo build --target wasm32-unknown-unknown -p ghostty-font \
  -p ghostty-config -p ghostty-terminal -p ghostty-vt -p ghostty-wasm
- cargo test -p ghostty-wasm (node runs the module against a fake canvas)

crates/ghostty-font/Cargo.toml

@@ -9,15 +9,23 @@ tiny-skia = "0.12.0"
 png = "0.18.1"
 thiserror = "2.0.21"
 log = "0.4.34"
-freetype-sys = "0.23.0"
-harfbuzz-sys = "0.8.0"
 ghostty-unicode.workspace = true
 bitflags = "2.13.2"
-yeslogic-fontconfig-sys = "6.0.1"
 ghostty-datastruct.workspace = true
 ghostty-config.workspace = true
 ghostty-terminal.workspace = true
 
+# The C font libraries are unavailable on wasm32, where the web_canvas
+# backend (browser Canvas API through JS imports) is used instead
+# (Zig: `font/backend.zig` `Backend.default`).
+[target.'cfg(not(target_arch = "wasm32"))'.dependencies]
+freetype-sys = "0.23.0"
+harfbuzz-sys = "0.8.0"
+yeslogic-fontconfig-sys = "6.0.1"
+
+[target.'cfg(target_arch = "wasm32")'.dependencies]
+ghostty-os.workspace = true
+
 # This crate wraps the freetype / harfbuzz / fontconfig C libraries through
 # their `-sys` crates; unsafe is denied by default and allowed on minimal,
 # `// SAFETY:`-commented items only.