diagnostics helpers for front-ends

I'm taking a look at beefing up the Stan mode in RStudio now. If I understand correctly, right now Stan produces compiler errors as text, e.g.

> rstan:::rstudio_stanc("~/scratch/test.stan")
SYNTAX ERROR, MESSAGE(S) FROM PARSER:

Warning (non-fatal): Comments beginning with # are deprecated.  Please use // in place of # for line comments.
Warning (non-fatal): Comments beginning with # are deprecated.  Please use // in place of # for line comments.
  error in 'model_test' at line 5, column 3
  -------------------------------------------------
     3: data {
     4:   int<lower=0> N
     5:   int<lower=0> nt[N]; 
          ^
     6:   int<lower=0> rt[N]; 
  -------------------------------------------------

PARSER EXPECTED: ";"
Error in stanc(model_code = paste(program, collapse = "\n"), model_name = model_cppname,  : 
  failed to parse Stan model 'test' due to the above error.

Does a similar API exist for getting diagnostic errors, perhaps as a list or data.frame of diagnostic errors? E.g. with each list entry containing a row, column, severity, and message, or similar. What I'd love to see is something like:

list(

  list(
    row = 1,
    column = 2,
    severity = "error",
    message = "parser expected ';'"
  ),

  < ... other diagnostics ... >
)

We could also just scrape the relevant information from the printed message, but having an explicit helper API would be useful.

Start by examining the rstan:::rstudio_stanc helper and the stanc diagnostic output shown in the issue. Determine how structured diagnostics could expose row, column, severity, and message, and consider the requested list or data.frame shape; done means a documented helper API can return those fields instead of requiring message scraping.