LibJS: Add left shift & right shift operator support for BigIntegers

Based on https://tc39.es/ecma262/#sec-numeric-types-bigint-leftShift
(This commit also includes the matching tests)

Userland/Libraries/LibJS/Tests/builtins/BigInt/bigint-basic.js

@@ -28,6 +28,8 @@ describe("correct behavior", () => {
         expect(1n | 2n).toBe(3n);
         expect(5n ^ 3n).toBe(6n);
         expect(~1n).toBe(-2n);
+        expect(5n << 2n).toBe(20n);
+        expect(7n >> 1n).toBe(3n);
     });
 
     test("increment operators", () => {