Multiline Problem Matcher

In #319 there was a discussion about the multiline "problem matcher" which was closed even though it was not actually fixed on the problem matcher side, so I think it's time to have this discussion again.

For example, OCaml has the following error log, which is very difficult to catch with current problem matcher.

File "bin/main.ml", line 0, characters 0-0:
type t =
  | Foo of string [@ocaml.warn_on_literal_pattern]
  | Bar of string

let no_warning = function
  | Bar "specific value" -> 0
  | _ -> 1

let warning = function
  | Foo "specific value" -> 0
  | _ -> 1
Warning 52 [fragile-literal-pattern]: Code should not depend on the actual values of
this constructor's arguments. They are only for information
and may change in future versions. (See manual section 13.5)

In such a case, it would be a fairly complex regular expression, but you can catch properly with code like the following, but the current implementation just works line-by-line.

^File\s"(.+)",\sline\s(\d+),\scharacters\s(\d+)-(\d+):\n((.|\n)+?)(Error|Warning)\s*(.*):\s((.|\n(?!File\s))+)$

https://regex101.com/r/gP6PEM/1

This means that it's impossible to matching such logs with such a regular expression.
In other words, we can't deal with issues like #319 on the user's end. Do you have any improvement plans for this?

No file or test is named. Start by reviewing the discussion in #319 and the current problem matcher behavior described in this issue, including the OCaml log and regex example; done should mean multiline diagnostics can be matched without breaking existing line-by-line matching.