Provide a `parse_transform` module to make path patterns easier to write and read

A caller could write something like that:
```erlang
khepri:put(
  StoreId,
  [wood,
   [Node || Node <- ?siblings,
            ?payload >= 50 andalso ?payload < 200
   ]])
```

This is just an idea, I didn’t think through it yet. It could look like something else. We just need to figure out the most intuitive syntax, if any.