feat(Util): add YELLOW to ColorResolvable (#3182)

src/util/Constants.js

@@ -422,6 +422,7 @@ exports.Colors = {
   AQUA: 0x1ABC9C,
   GREEN: 0x2ECC71,
   BLUE: 0x3498DB,
+  YELLOW: 0xFFFF00,
   PURPLE: 0x9B59B6,
   LUMINOUS_VIVID_PINK: 0xE91E63,
   GOLD: 0xF1C40F,

src/util/Util.js

@@ -251,6 +251,7 @@ class Util {
    * - `AQUA`
    * - `GREEN`
    * - `BLUE`
+   * - `YELLOW`
    * - `PURPLE`
    * - `LUMINOUS_VIVID_PINK`
    * - `GOLD`