Restore eof_reached

Signed-off-by: Rudi Grinberg <rudi.grinberg@gmail.com>

src/dune_lexer.mli

@@ -12,3 +12,5 @@ val first_line : Lexing.lexbuf -> first_line
 (** Parse the first line of a versioned file but do not fail if it
     doesn't start with [(lang ...)]. *)
 val maybe_first_line : Lexing.lexbuf -> first_line option
+
+val eof_reached : Lexing.lexbuf -> bool

src/dune_lexer.mll

@@ -61,6 +61,10 @@ and to_eol = parse
     { ()
     }
 
+and eof_reached = parse
+  | eof { true  }
+  | ""  { false }
+
 {
   let first_line lb =
     match maybe_first_line lb with