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/av_from_text.R

@@ -168,7 +168,7 @@ av_from_text <- function(text,
       }
     })
   } else {
-    stop_("`type` must be either 'drug', 'dose' or 'administration'")
+    stop_("{.arg type} must be either 'drug', 'dose' or 'administration'")
   }
 
   # collapse text if needed