VideoPress: consider a way to add a "Clean custom size" option

Sometimes gets tricky to remove the custom size of the block, especially when it's combined with some alignments. Maybe we'd like to add an option to clean the custom size.

No files or tests are named. Start by locating the VideoPress block's custom-size and alignment controls, then trace how the current size is stored and cleared. Done means users have a clear option to remove the custom size without breaking the block's alignment behavior.

Written by the indexing model from the issue text.