Write lexer to parse %{deps[i]}

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

src/dune_vars.mli

@@ -0,0 +1 @@
+val deps : string -> int option

src/dune_vars.mll

@@ -0,0 +1,13 @@
+{
+  open Stdune
+}
+
+let digit     = ['0'-'9']
+
+rule deps = parse
+  | "deps[" (digit+ as i) "]" eof { Some (Int.of_string_exn i) }
+  | _  { None }
+
+{
+let deps s = deps (Lexing.from_string s)
+}