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/aa_helper_functions.R

@@ -382,6 +382,14 @@ pkg_is_available <- function(pkg, also_load = FALSE, min_version = NULL) {
   isTRUE(out)
 }
 
+highlight_code <- function(code) {
+  if (pkg_is_available("cli", min_version = "3.0.0")) {
+    cli::code_highlight(code)
+  } else {
+    code
+  }
+}
+
 import_fn <- function(name, pkg, error_on_fail = TRUE) {
   if (isTRUE(error_on_fail)) {
     stop_ifnot_installed(pkg)
@@ -396,7 +404,7 @@ import_fn <- function(name, pkg, error_on_fail = TRUE) {
     getExportedValue(name = name, ns = asNamespace(pkg)),
     error = function(e) {
       if (isTRUE(error_on_fail)) {
-        stop_("function `", name, "()` is not an exported object from package '", pkg,
+        stop_("function {.code ", name, "()} is not an exported object from package '", pkg,
           "'. Please create an issue at https://github.com/msberends/AMR/issues. Many thanks!",
           call = FALSE
         )