(fix) awaited type of promise or plain value (#439)

When awaiting the value of a type like Promise<string> | null, the type
wouldn't match PromiseLike. Hence the awaited value would still be
Promise<string> | null and not string | null. Reducing it to one type
declaration and using a conditional type to infer the awaited type
should do the trick.

Author: yagebu

packages/svelte2tsx/svelte-shims.d.ts

@@ -154,15 +154,10 @@ declare function __sveltets_unionType<T1, T2, T3>(t1: T1, t2: T2, t3: T3): T1 |
 declare function __sveltets_unionType<T1, T2, T3, T4>(t1: T1, t2: T2, t3: T3, t4: T4): T1 | T2 | T3 | T4;
 declare function __sveltets_unionType(...types: any[]): any;
 
-declare function __sveltets_awaitThen<T>(
-    promise: PromiseLike<T>,
-    onfulfilled: (value: T) => any,
-    onrejected?: (value: any) => any
-): any;
 declare function __sveltets_awaitThen<T>(
     promise: T,
-    onfulfilled: (value: T) => any,
-    onrejected?: (value: never) => any
+    onfulfilled: (value: T extends PromiseLike<infer U> ? U : T) => any,
+    onrejected?: (value: T extends PromiseLike<any> ? any : never) => any
 ): any;
 
 declare function __sveltets_each<T>(