chore: add lexer.mli

Signed-off-by: Rudi Grinberg <me@rgrinberg.com>

ps-id: 65439b6f-a4a5-4f38-bab3-58d2e59e65e2

otherlibs/ocamlc_loc/src/lexer.mli

@@ -0,0 +1,44 @@
+exception Unknown_format
+
+type lines =
+  | Single of int
+  | Range of int * int
+
+type source =
+  | Code of
+      { code : int
+      ; name : string
+      }
+  | Alert of string
+
+type severity =
+  | Error of source option
+  | Warning of source
+
+type loc =
+  { chars : (int * int) option
+  ; lines : lines
+  ; path : string
+  }
+
+type line =
+  { indent : int
+  ; contents : string
+  }
+
+type token =
+  | Toplevel of
+      { indent : int
+      ; loc : loc
+      ; severity : severity
+      ; message : string
+      }
+  | Related of
+      { indent : int
+      ; loc : loc
+      ; message : string
+      }
+  | Line of line
+  | Eof
+
+val token : Lexing.lexbuf -> token