Add check for installed box.lsp package in .Rprofile

[radbasa]

Have you read the Contributing Guidelines?

Issue #28

How to test

1. Run box.lsp::use_box_lsp() to set up .Rprofile.
2. With box.lsp installed, test that auto-complete works.
3. remove.packages("box.lsp")
4. Restart R session or restart VSCode.
5. Test that auto-complete does not work.

Definition of Done

• The change is thoroughly documented.
• The CI passes (R CMD check, linter, unit tests, spelling).
• Any generated files have been updated (e.g. .Rd files with roxygen2::roxygenise())