Add a way to filter blocks

As of now, the `blocks()` iterator method takes in one parameter: `layer` (for i.e. waterlogged blocks)

It will always return all blocks in a layer, but one may desire to only get all `planks` or `stone` blocks, or even all liquids (still and flowing lava & water). There can also be the opposite case where you want to ignore specific blocks, i.e. `air` or `structure_void`.

There are multiple ways to face this:
- Add an additional `filter` parameter, taking in a BlockPalette, and another parameter for ignoring unwanted blocks
- A parameter taking in a string. This would allow for having wildcards for metadata. The strings might break due to block or property renames though.
- A parameter taking in a Filter object. This is likely a flexible solution, whilst still easy to use.
- A parameter taking in a `Closure` or `callable` to allow the developer to write their own filters