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/src/shaper/mod.rs

@@ -1,11 +1,18 @@
 //! Text shapers. Port of `font/shaper/`.
 
+#[cfg(not(target_arch = "wasm32"))]
 pub mod cache;
 pub mod feature;
+#[cfg(not(target_arch = "wasm32"))]
 pub mod harfbuzz;
+#[cfg(not(target_arch = "wasm32"))]
 mod hb;
+#[cfg(not(target_arch = "wasm32"))]
 pub mod noop;
+#[cfg(not(target_arch = "wasm32"))]
 pub mod run;
 
-#[cfg(test)]
+pub mod web_canvas;
+
+#[cfg(all(test, not(target_arch = "wasm32")))]
 pub(crate) mod test_util;