Optimizing parsing with global options

[coatless]

From @mpadge on rOpenSci Slack:

My only real guide was and remains https://roxygen2.r-lib.org/reference/roclet.html. It seems what you need is initial evaluation via roclet_preprocess which knows nothing of webR URL, then extraction from DESC, and eventual use of URL and block evaluation in roclet_process. That's the flow of srr, because process also does loads of complex things including grabbing external objects for validation which are clearly out of the scope of pre_process / tag_parse.

Example: https://github.com/ropensci-review-tools/srr/blob/main/R/roclet.R

I'm going to try to tackle this in another PR later this week. This will be more involved to change the underlying parsing mechanics. If anything, I think a new article on roclets is in order.

Originally posted by @coatless in #1 (comment)