[R] [ci] move linting code out of package

[jameslamb]

Contributes to #8012.

This PR proposes the following changes to the way this project uses {lintr} to lint its R code.

• moves the run_lint.R out of the package and up into tests/ci_build/
  • this script is only used during development, so it doesn't need to be included in the package distribution
  • doing this reduces the size of the built package on CRAN, and removes two Suggests dependencies ({lintr} and {crayon}), reducing the effort required for those other projects to test new releases
• modifies the linting code so that .Rmd files are also checked
• completely removes use of {crayon} in the linting code
  • this is just used to color terminal output, which I think is not worth the risk of CI builds that could be caused by failure to install that library + the added CI time to download and install it

How I tested this

Rscript ./tests/ci_build/lint_r.R "$(pwd)"

This shows 1285 linting errors, which II'm planning to propose fixes for in follow-up PRs.

Notes for Reviewers

Thanks for your time and consideration.

[trivialfis]

Thank you for working on this! Question inlined in comments.

Also, apologies for the failing CI. GitHub action seems to be breaking down ...