A block is analogous to a CSS block in most cases. A single SCSS document
is encapsulated in a block when parsed, but it does not have parent tags
so all of its children appear on the root level when compiled.
Blocks are made up of selectors and children.
The children of a block are just all the blocks that are defined within.
Compiling the block involves pushing a fresh environment on the stack,
and iterating through the props, compiling each one.