Added semantics for pairing blocks together

Added semantics for pairing blocks together

This required us to modify how the parser works so that once it
detects a pair of blocks, it kicks it back to our specific function
which allows us to detect if the pair of blocks it detected were a
matching pair. This is required in order to allow single blocks to
be included within paired blocks, as otherwise it would always match
the last single block to the end block.

This required changing the grammar so the pair blocks had their own
named expression. This allows us to reject the parse as invalid with
incorrect semantics and allows it to try to just parse the first
block alone.