Add Web export presets and update Rust configuration for Emscripten

rust/.cargo/config.toml

@@ -0,0 +1,20 @@
+[target.wasm32-unknown-emscripten]
+rustflags = [
+    "-C",
+    "link-args=-pthread",                            # Required for Thread Support
+    "-C",
+    "target-feature=+atomics",                       # Required for Thread Support
+    "-C",
+    "link-args=-sSIDE_MODULE=2",                     # Compiles as a dynamic library for Godot to load
+    "-C",
+    "llvm-args=-enable-emscripten-cxx-exceptions=0",
+    "-Z",
+    "default-visibility=hidden",                     # Prevents symbol conflicts
+    "-Z",
+    "link-native-libraries=no",
+    "-Z",
+    "emscripten-wasm-eh=false",
+]
+
+[env]
+GODOT4_BIN = { value = "/usr/bin/godot", force = true }