feat: Add support for custom MP4 metadata with mdta format (#132)

* feat: Add support for custom MP4 metadata with mdta format

- Add metadataFormat option ('mdir' | 'mdta') to IsobmffOutputFormatOptions
- Implement complete mdta format with keys box generation
- Enable custom metadata fields readable by ffmpeg/ffprobe
- Maintain backward compatibility with default mdir format

* Refactor ISOBMFF metadata tag writing logic

* Autism

---------

Co-authored-by: tanfu <[email protected]>
Co-authored-by: Vanilagy <[email protected]>

dev/convert.html

@@ -24,7 +24,9 @@
 			chunked: true,
 			chunkSize: 2**20
 		});
-		const outputFormat = new Mediabunny.FlacOutputFormat({});
+		const outputFormat = new Mediabunny.Mp4OutputFormat({
+			metadataFormat: 'mdta',
+		});
 
 		const button = document.createElement('button');
 		button.textContent = 'Cancel';
@@ -92,7 +94,7 @@
 			},
 			*/
 			video: () => ({
-				discard: true,
+				//discard: true,
 				//discard: true,
 				//crop: {
 				//	left: 0,
@@ -126,7 +128,7 @@
 				//width: 200,
 				//height: 100,
 			}),
-			tags: {} ?? {
+			tags: {
 				title: 'Bigggy',
 				artist: 'Buck Bunny',
 				images: [{