ci: replace auto styler with pr-commands

[dshemetov]

Checklist

Please:

• Make sure this PR is against "dev", not "main".
• Request a review from one of the current epipredict main reviewers:
  dajmcdon.
• Make sure to bump the version number in DESCRIPTION and NEWS.md.
  Always increment the patch version number (the third number), unless you are
  making a release PR from dev to main, in which case increment the minor
  version number (the second number).
• Describe changes made in NEWS.md, making sure breaking changes
  (backwards-incompatible changes to the documented interface) are noted.
  Collect the changes under the next release number (e.g. if you are on
  0.7.2, then write your changes under the 0.8 heading).
• Consider pinning the epiprocess version in the DESCRIPTION file if
  • You anticipate breaking changes in epiprocess soon
  • You want to co-develop features in epipredict and epiprocess

Change explanations for reviewer

So our automated CI actions have felt more annoying to me than helpful lately. Here's a quick change to make these actions opt-in, rather than default on:

• Remove the automated styler command that tries to fix stuff (and often breaks if you push too many commits at once)
• Replace it with the "pr-commands" workflow, which similar to the "/preview-docs" command, will run styler or document in response to "/style" and "/document" (this action is preferred in tidyverse/dplyr for instance)

If it seems like a good idea, let's do it, if not, feel free too close, probably not worth too much deliberation.

Magic GitHub syntax to mark associated Issue(s) as resolved when this is merged into the default branch

• Resolves #{issue number}