refactor: add signature_help mli (#913)

Signed-off-by: Rudi Grinberg <[email protected]>

ocaml-lsp-server/src/signature_help.mli

@@ -0,0 +1,23 @@
+open Merlin_kernel
+open Ocaml_parsing
+open Ocaml_typing
+
+type parameter_info =
+  { label : Asttypes.arg_label
+  ; param_start : int
+  ; param_end : int
+  ; argument : Typedtree.expression option
+  }
+
+type application_signature =
+  { function_name : string option
+  ; function_position : Msource.position
+  ; signature : string
+  ; parameters : parameter_info list
+  ; active_param : int option
+  }
+
+val application_signature :
+     prefix:string
+  -> ('a * Merlin_specific.Browse_raw.node) list
+  -> application_signature option