New bullet styles for TODOs and questions

Fixes #779. Also touches on the subject matter of the usethis vignette ["Converting usethis's UI to use cli"](https://usethis.r-lib.org/articles/ui-cli-conversion.html) because of the TODOs. Proposed new syntax: ``` r cli_bullets(c("?" = "This is a question", "[]" = "This is a TODO")) #> ❓ This is a question #> ☐ This is a TODO ``` Some outstanding questions: - Do we want to use the fancy question mark instead of the regular one? - Is red the correct colour? It felt correct to me for the TODO given that `usethis::ui_todo()` used red as well. - The usethis vignette proposes the underscore `"_"` as the LHS for TODO instead of `"[]"`. The `"[]"` feels more natural to me, but it is two characters whereas all the others are one. Other things. - Using roxygen2 (even the development version) seems to be broken for me because of a problem unrelated to the PR. It's in the "cliapp-docs.R".