Add bitdepth option to heif output (#4036)

Prebuilt binaries support only AVIF with a bitdepth of 8

test/unit/heif.js

@@ -78,4 +78,14 @@ describe('HEIF', () => {
       sharp().heif({ compression: 'av1', chromaSubsampling: '4:4:4' });
     });
   });
+  it('valid bitdepth value does not throw an error', () => {
+    assert.doesNotThrow(() => {
+      sharp().heif({ compression: 'av1', bitdepth: 12 });
+    });
+  });
+  it('invalid bitdepth value should throw an error', () => {
+    assert.throws(() => {
+      sharp().heif({ compression: 'av1', bitdepth: 11 });
+    }, /Error: Expected 8, 10 or 12 for bitdepth but received 11 of type number/);
+  });
 });