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.R

@@ -1277,8 +1277,8 @@ as_sir_method <- function(method_short,
   }
   if (is.null(mo)) {
     stop_("No information was supplied about the microorganisms (missing argument {.arg mo} and no column of class 'mo' found). See {.help AMR::as.sir}().\n\n",
-      "To transform certain columns with e.g. mutate(), use `data %>% mutate(across(..., as.sir, mo = x))`, where x is your column with microorganisms.\n",
-      "To transform all ", method_long, " in a data set, use `data %>% as.sir()` or `data %>% mutate_if(is.", method_short, ", as.sir)`.",
+      "To transform certain columns with e.g. mutate(), use ", highlight_code("data %>% mutate(across(..., as.sir, mo = x))"), ", where x is your column with microorganisms.\n",
+      "To transform all ", method_long, " in a data set, use ", highlight_code("data %>% as.sir()"), " or ", highlight_code(paste0("data %>% mutate_if(is.", method_short, ", as.sir)")), ".",
       call = FALSE
     )
   }