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/shape.rs

@@ -3,12 +3,17 @@
 //! The shaper implementation for the Linux backend is HarfBuzz
 //! ([`crate::shaper::harfbuzz::Shaper`]).
 
+pub use crate::shaper::feature::{DEFAULT_FEATURES, Feature, FeatureList};
 /// Shaper implementation for our compile options (backend
 /// `fontconfig_freetype` => HarfBuzz).
+#[cfg(not(target_arch = "wasm32"))]
 pub use crate::shaper::harfbuzz::Shaper;
+/// Shaper implementation for the `web_canvas` backend (wasm32).
+#[cfg(target_arch = "wasm32")]
+pub use crate::shaper::web_canvas::Shaper;
+#[cfg(not(target_arch = "wasm32"))]
 pub use crate::shaper::{
     cache::Cache,
-    feature::{DEFAULT_FEATURES, Feature, FeatureList},
     harfbuzz,
     noop,
     run::{RunIterator, RunIteratorHook, RunOptions, TextRun},