⚠️Disclaimer⚠️ this PR implements syntax that is still in an open RFC. It will be adjusted once the RFC is closed.
These changes implement the `BlockGroup` and `Block` AST nodes that will then be used to generate instructions based on the new syntax. A `BlockGroup` is a container for `Block` instances. The first block of a block is always implicit and required while any subsequent blocks are optional.
PR Close#50953