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/mo_property.R

@@ -1043,10 +1043,10 @@ find_mo_col <- function(fn) {
   )
   if (!is.null(df) && !is.null(mo) && is.data.frame(df)) {
     if (message_not_thrown_before(fn = fn)) {
-      message_("Using column '", font_bold(mo), "' as input for `", fn, "()`")
+      message_("Using column '", font_bold(mo), "' as input for {.help AMR::", fn, "}()")
     }
     return(df[, mo, drop = TRUE])
   } else {
-    stop_("argument `x` is missing and no column with info about microorganisms could be found.", call = -2)
+    stop_("argument {.arg x} is missing and no column with info about microorganisms could be found.", call = -2)
   }
 }