Add Prettier config for JS/TS/HTML formatting

This helps ensure a consistent code style and helps editors like Zed
that do LSP format-on-save by default.

.prettierrc

@@ -0,0 +1,7 @@
+{
+	"useTabs": true,
+	"tabWidth": 4,
+	"semi": true,
+	"singleQuote": true,
+	"printWidth": 150,
+}

Makefile

@@ -107,6 +107,9 @@ dev: $(DEMO)
 format:
 	clang-format -i $(LIBRARY_SOURCES) $(DEMO_SOURCES) $(GLTFPACK_SOURCES)
 
+formatjs:
+	prettier -w js/*.js gltf/*.js demo/*.html js/*.ts
+
 js: js/meshopt_decoder.js js/meshopt_decoder.module.js js/meshopt_encoder.js js/meshopt_encoder.module.js js/meshopt_simplifier.js js/meshopt_simplifier.module.js
 
 symbols: $(BUILD)/amalgamated.so