StringSlice: Fix return type when passing in non-literal (#1036)

Author: ghaaj

source/string-slice.d.ts

@@ -31,7 +31,7 @@ export type StringSlice<
 	Start extends number = 0,
 	End extends number = StringToArray<S>['length'],
 > = string extends S
-	? string[]
+	? string
 	: ArraySlice<StringToArray<S>, Start, End> extends infer R extends readonly string[]
 		? Join<R, ''>
 		: never;

test-d/string-slice.ts

@@ -12,3 +12,6 @@ expectType<StringSlice<'abcde', -100, -3>>('ab');
 expectType<StringSlice<'abcde', 3, 100>>('de');
 expectType<StringSlice<'abcde', 1, 1>>('');
 expectType<StringSlice<'abcde', 100, 1>>('');
+expectType<StringSlice<string>>(null! as string);
+expectType<StringSlice<string, 1>>(null! as string);
+expectType<StringSlice<string, 1, 2>>(null! as string);