refactor(mw.Map): use generics

Avoid passing types when using `mw.messages.exists<types>()` and `mw.messages.set<types>()` to prevent the error "An_instantiation_expression_cannot_be_followed_by_a_property_access"

Copied from AnYiEE@6df693a by @AnYiEE

mw/Map.d.ts

@@ -27,7 +27,7 @@ declare global {
              * @returns True if the key exists
              * @see https://doc.wikimedia.org/mediawiki-core/master/js/#!/api/mw.Map-method-exists
              */
-            exists(selection: keyof V): boolean;
+            exists<S extends keyof V>(selection: S): selection is S;
 
             /**
              * Get the value of one or more keys.
@@ -53,7 +53,7 @@ declare global {
              * @see https://doc.wikimedia.org/mediawiki-core/master/js/#!/api/mw.Map-method-set
              */
             set<S extends keyof V>(selection: S, value: V[S]): boolean;
-            set(selection: Partial<V>): boolean;
+            set<S extends Partial<V>>(selection: S): boolean;
         }
     }
 }