frontend: Add missing new file

Forgot to add the new ValidityUnit.ts file to the prev commit.

Fixes: 1c2fb50

Author: Emily Ehlert

frontend/src/types/ValidityUnit.ts

@@ -0,0 +1,6 @@
+export enum ValidityUnit {
+    Year = 0,
+    Month = 1,
+    Day = 2,
+    Hour = 3
+}