Add generic type parameter to startsWith/endsWith

Author: LinusU

src/lib/es2015.core.d.ts

@@ -401,7 +401,7 @@ interface String {
      * same as the corresponding elements of this object (converted to a String) starting at
      * endPosition – length(this). Otherwise returns false.
      */
-    endsWith(searchString: string, endPosition?: number): boolean;
+    endsWith<T extends string>(searchString: T, endPosition?: number): this is `${string}${T}`;
 
     /**
      * Returns the String value result of normalizing the string into the normalization form
@@ -431,7 +431,7 @@ interface String {
      * same as the corresponding elements of this object (converted to a String) starting at
      * position. Otherwise returns false.
      */
-    startsWith(searchString: string, position?: number): boolean;
+    startsWith<T extends string>(searchString: T, position?: number): this is `${T}${string}`;
 
     /**
      * Returns an `<a>` HTML anchor element and sets the name attribute to the text value