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/custom_antimicrobials.R

@@ -155,7 +155,7 @@ add_custom_antimicrobials <- function(x) {
 
   AMR_env$ab_previously_coerced <- AMR_env$ab_previously_coerced[which(!AMR_env$ab_previously_coerced$ab %in% c(x$ab, x$generalised_name) & !AMR_env$ab_previously_coerced$x %in% c(x$ab, x$generalised_name)), , drop = FALSE]
   class(AMR_env$AB_lookup$ab) <- c("ab", "character")
-  message_("Added ", nr2char(nrow(x)), " record", ifelse(nrow(x) > 1, "s", ""), " to the internal `antimicrobials` data set.")
+  message_("Added ", nr2char(nrow(x)), " record", ifelse(nrow(x) > 1, "s", ""), " to the internal {.code antimicrobials} data set.")
 }
 
 #' @rdname add_custom_antimicrobials