fix detection of project.xml to allow an id with backslashes

While the plugin always resolves with forward slashes, Rollup might give the initial project.xml backslashes (but Vite doesn't seem to)

Author: joshtynjala

vite-plugin-openfl.mjs

@@ -51,7 +51,7 @@ export default function openflPlugin() {
       config = resolvedConfig;
     },
     resolveId(source, importer) {
-      if (!/\/project\.xml$/.test(importer)) {
+      if (!/[\/\\]project\.xml$/.test(importer)) {
         return;
       }
 
@@ -79,7 +79,7 @@ export default function openflPlugin() {
       }
     },
     transform(code, id, options) {
-      if (!/\/project\.xml$/.test(id)) {
+      if (!/[\/\\]project\.xml$/.test(id)) {
         return;
       }