Add a WASM build configuration

The config.toml lets me run `cargo run --target wasm32-unknown-unknown`
and host a dev server.

I've adjusted the Cargo.toml to use the kdtree_rayon feature by default,
but disable the Rayon feature for WASM builds (because it doesn't work).

.cargo/config.toml

@@ -0,0 +1,2 @@
+[target.wasm32-unknown-unknown]
+runner = "wasm-server-runner"

Cargo.toml

@@ -5,7 +5,6 @@ edition = "2024"
 
 [dependencies]
 bevy = "0.16.0"
-bevy_spatial = "0.11.0"
 
 # Grand-dependency pins
 ab_glyph = "0.2.16"
@@ -17,6 +16,16 @@ miniz-sys = "0.1.10"
 nonmax = "0.5.1"
 rand = "0.8.0"
 
+# Use regular bevy_spatial on non-WASM builds
+[target.'cfg(not(target_arch = "wasm32"))'.dependencies]
+bevy_spatial = "0.11.0"
+
+# Use bevy_spatial *without* the kdtree_rayon feature when building for WASM.
+[target.'cfg(target_arch = "wasm32")'.dependencies.bevy_spatial]
+version = "0.11.0"
+default-features = false
+features = ["kdtree"]
+
 [profile.dev]
 opt-level = 1