Add String_with_vars.text_only

Signed-off-by: Jeremie Dimino <jeremie@dimino.org>

src/string_with_vars.ml

@@ -272,3 +272,8 @@ let is_var { template; syntax_version = _ } ~name =
   match template.parts with
   | [Var n] -> name = Var.full_name n
   | _ -> false
+
+let text_only t =
+  match t.template.parts with
+  | [Text s] -> Some s
+  | _ -> None

src/string_with_vars.mli

@@ -30,6 +30,9 @@ val virt_text  : (string * int * int * int) -> string -> t
 
 val is_var : t -> name:string -> bool
 
+(** If [t] contains no variable, returns the contents of [t]. *)
+val text_only : t -> string option
+
 module Mode : sig
   type 'a t =
     | Single : Value.t t