Formatting indentation for blocks

Imagine the following rules

Block:
	'block' name=ID ':'
	BEGIN
		blockStatements+=BlockStatement*
	END
	'endblock'
;

terminal BEGIN: 'synthetic:BEGIN';
terminal END: 'synthetic:END';

I tried many things in order to format that code so that every statement inside the block is indented, but I cannot make it work. I tried block.interior[indent], which I would have thought should do the trick.

Start by reproducing the formatting behavior with the grammar shown in the issue, focusing on the Block and blockStatements rules and the attempted block.interior[indent] configuration. Read the formatter documentation and relevant implementation to determine how indentation for synthetic BEGIN and END terminals is handled; done means statements inside the block are consistently indented.