Add natural blocks as first class concepts to P and P

[breck7]

A pattern that has proved very useful in Scroll is blocks. We can upstream that to the Particles libary.
Blocks are, sequences of particles separated by blank particles.

this is
a block

and this is another block

Some methods might be:

get blocks() {}

We could also add block parsing logic to Parsers. The default would be all parsers in scope for all blocks. But then we could have something like blockScope 0, for top matter blocks, etc