Add support for ~U sigil (#442)

`~U` has been recently introduced. See elixir-lang/elixir#8821 for context.

Author: rranelli

elixir-mode.el

@@ -164,7 +164,7 @@
  (or "_" "__MODULE__" "__DIR__" "__ENV__" "__CALLER__"
  "__block__" "__aliases__")
  symbol-end))
- (sigils . ,(rx "~" (or "B" "C" "D" "N" "R" "S" "T" "b" "c" "r" "s" "w")))))
+ (sigils . ,(rx "~" (or "B" "C" "D" "N" "R" "S" "T" "U" "b" "c" "r" "s" "w")))))
 
  (defmacro elixir-rx (&rest sexps)
  (let ((rx-constituents (append elixir-rx-constituents rx-constituents)))