Add natural blocks as first class concepts to P and P

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

Start by reading the Particles and Parsers implementations and compare how Scroll currently represents blocks. Clarify the block model, parser scope behavior, and the expected public methods before identifying tests; done means the agreed block concepts and parsing behavior are implemented and covered by tests.

Written by the indexing model from the issue text.