Implement cli::code_highlight() for R code examples in messages (issue #191)

Add highlight_code() helper that wraps cli::code_highlight() when cli >= 3.0.0
is available, falling back to plain code otherwise. Apply it to all inline
R code examples embedded in message/warning/stop strings across the package.

Also convert remaining backtick-quoted function and argument references in
messaging calls to proper cli markup: {.help AMR::fn}(), {.arg arg},
{.code expr}, and {.pkg pkg} throughout ab.R, ab_from_text.R, av_from_text.R,
amr_selectors.R, count.R, custom_antimicrobials.R, custom_microorganisms.R,
interpretive_rules.R, mo.R, mo_property.R, sir.R, sir_calc.R.

Fixes #191

https://claude.ai/code/session_01XHWLohiSTdZvCutwD7ag2b

R/amr_selectors.R

@@ -837,7 +837,7 @@ amr_select_exec <- function(function_name,
 #' @export
 #' @noRd
 print.amr_selector <- function(x, ...) {
-  warning_("It should never be needed to print an antimicrobial selector class. Are you using data.table? Then add the argument `with = FALSE`, see our examples at `?amr_selector`.",
+  warning_("It should never be needed to print an antimicrobial selector class. Are you using {.pkg data.table}? Then add the argument {.code with = FALSE}, see our examples at {.help AMR::amr_selector}().",
     immediate = TRUE
   )
   cat("Class 'amr_selector'\n")
@@ -1062,7 +1062,7 @@ message_agent_names <- function(function_name, agents, ab_group = NULL, examples
   if (message_not_thrown_before(function_name, sort(agents))) {
     if (length(agents) == 0) {
       if (is.null(ab_group)) {
-        message_("For `", function_name, "()` no antimicrobial drugs found", examples, ".")
+        message_("For {.help AMR::", function_name, "}() no antimicrobial drugs found", examples, ".")
       } else if (ab_group == "administrable_per_os") {
         message_("No orally administrable drugs found", examples, ".")
       } else if (ab_group == "administrable_iv") {