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-terminal/Cargo.toml

@@ -12,11 +12,15 @@ ghostty-datastruct.workspace = true
 ghostty-unicode.workspace = true
 ghostty-vt.workspace = true
 log = "0.4.34"
+png = "0.18.1"
+thiserror = "2.0.21"
+
+# C / POSIX-only dependencies are unavailable on wasm32 (Zig builds the wasm
+# module without oniguruma, and kitty shared-memory images need POSIX shm).
+[target.'cfg(not(target_arch = "wasm32"))'.dependencies]
 onig = "6.5.3"
 ghostty-onig.workspace = true
-png = "0.18.1"
 rustix = { version = "1.1.5", features = ["shm", "fs"] }
-thiserror = "2.0.21"
 
 [dev-dependencies]
 ghostty-config.workspace = true