Find the groups of expressions that should be processed together

Usage

cache_find_block(pd)

We want blocks to be formed according to these rules:

Blocks should contain either cached or uncached expressions only. If a block contains cached expressions only, it does not have to be processed and can be returned immediately. If a block contains uncached expressions, it makes sense to put as many uncached expression in it, since processing one bigger block has less overhead than processing many smaller blocks.
Multiple expressions can sit on one row, e.g. in-line comment and commands separated with ";". This creates a problem when processing each expression separately because when putting them together, we need complicated handling of line breaks between them, as it is not a priori clear that there is a line break separating them. To avoid this, we put top-level expressions that sit on the same line into one block, so the assumption that there is a line break between each block of expressions holds.
All expressions in a stylerignore sequence must be in the same block. If that's not the case, the first expression in a block might not be a top-level terminal, but another top-level expression. apply_stylerignore() joins env_current$stylerignore, which contains only terminals, with the first expression in a stylerignore sequence, based on the first pos_id in that stylerignore sequence (first_pos_id_in_segment).