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/metrics.rs

@@ -241,9 +241,8 @@ fn zround(v: f64) -> f64 {
 
 impl Metrics {
     /// All-zero metrics, except `cursor_thickness` which defaults to 1.
-    /// (Zig's private `init()`; used by tests.)
-    #[cfg(test)]
-    fn init() -> Metrics {
+    /// (Zig's private `init()`; used by tests and the web_canvas face.)
+    pub(crate) fn init() -> Metrics {
         Metrics {
             cell_width: 0,
             cell_height: 0,