Add util and rewrite examples (WIP)

apps/examples/node/concatDemuxer.js

@@ -0,0 +1,13 @@
+const fs = require('fs');
+const { createFFmpeg, fetchFile } = require('../../src');
+
+const ffmpeg = createFFmpeg({ log: true });
+
+(async () => {
+  await ffmpeg.load();
+  ffmpeg.FS('writeFile', 'flame.avi', await fetchFile('../assets/flame.avi'));
+  ffmpeg.FS('writeFile', 'concat_list.txt', 'file flame.avi\nfile flame.avi');
+  await ffmpeg.run('-f', 'concat', '-safe', '0', '-i', 'concat_list.txt', 'flame.mp4');
+  await fs.promises.writeFile('flame.mp4', ffmpeg.FS('readFile', 'flame.mp4'));
+  process.exit(0);
+})();