[compat] Implement compat.imghdr

Python 3.11 deprecates `imghdr` module

yt_dlp/compat/imghdr.py

@@ -0,0 +1,14 @@
+tests = {
+    'webp': lambda h: h[0:4] == b'RIFF' and h[8:] == b'WEBP',
+    'png': lambda h: h[:8] == b'\211PNG\r\n\032\n',
+    'jpeg': lambda h: h[6:10] in (b'JFIF', b'Exif'),
+}
+
+
+def what(path):
+    """Detect format of image (Currently supports jpeg, png, webp only)
+    Ref: https://github.com/python/cpython/blob/3.10/Lib/imghdr.py
+    """
+    with open(path, 'rb') as f:
+        head = f.read(12)
+    return next((type_ for type_, test in tests.items() if test(head)), None)