Use a port other than 8000 for npm serve.

If the port is already open by a different appliation, esbuild reports
no error and does not switch to another port. Instead, the URL it prints
displays an unrelated page.

This change should reduce the likelihood of collision.

build.mjs

@@ -53,7 +53,7 @@ if (mode === 'build' || mode === 'minify') {
     await context.rebuild();
     await context.watch();
     // Specifying `servedir` is necessary for files built by meta URL plugin to be accessible.
-    await context.serve({ servedir: 'dist' });
+    await context.serve({ servedir: 'dist', port: 8010 });
 } else {
     console.error(`Usage: ${process.argv0} [build|watch|serve|minify]`);
 }