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/sir_calc.R

@@ -173,7 +173,7 @@ sir_calc <- function(...,
   if (print_warning == TRUE) {
     if (message_not_thrown_before("sir_calc")) {
       warning_("Increase speed by transforming to class 'sir' on beforehand:\n",
-        "  your_data %>% mutate_if(is_sir_eligible, as.sir)",
+        highlight_code("  your_data %>% mutate_if(is_sir_eligible, as.sir)"),
         call = FALSE
       )
     }