libghostty: simplify Wasm allocation API

Replace a bunch of type-specific Wasm allocation functions with a generic
byte allocator and reusable opaque out-parameters for pointers. This
makes it a lot more ergonomic (relatively) to use the Wasm interface
and removes a dozen or so exports.

This also updates the `ghostty_type_json` `abi` field with a maximum
alignment value that host sides can use to keep every allocation aligned
properly, easily, without hardcoding numbers.

This adds a test to verify this all works as intended and runs in CI.

.github/workflows/test.yml

@@ -733,6 +733,11 @@ jobs:
             echo "Verified ${artifact} requires simd128"
           done
 
+      - name: Test WASM allocator
+        run: |
+          nix develop -c node test/wasm-alloc.mjs ghostty-vt.wasm
+          nix develop -c node test/wasm-alloc.mjs ghostty-vt-small.wasm
+
   # Compile-only checks for the -Dvt-features flags so that future changes
   # don't regress any feature combination.
   build-libghostty-vt-features: