This is a major change to the way block metadata is handled within the PM core. This separates variant metadata (which really ought to be part of the ID) from state metadata, and in a couple of cases flattens separate states of blocks together.
The result of this is that invalid variants can be much more easily detected, and additionally state handling is much cleaner since meta is only needed at the serialize layer instead of throughout the code.
The original regex almost completely failed at its objective, because it a) only worked if there was no value for the key, and b) did not prevent all such occurrences getting transformed, while quoting patterns that would not get transformed anyway.
