LibJS+LibCrypto: Fix SignedBitInteger::bitwise_not and use it in LibJS

Bitwise operators are defined on two's complement, but SignedBitInteger
uses sign-magnitude. Correctly convert between the two.

Let LibJS delegate to SignedBitInteger for bitwise_not, like it does
for all other bitwise_ operations on bigints.

No behavior change (LibJS is now the only client of
SignedBitInteger::bitwise_not()).

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

@@ -40,6 +40,8 @@ describe("correct behavior", () => {
         expect(1n | 2n).toBe(3n);
         expect(5n ^ 3n).toBe(6n);
         expect(~1n).toBe(-2n);
+        expect(~-1n).toBe(0n);
+
         expect(5n << 2n).toBe(20n);
         expect(7n >> 1n).toBe(3n);
     });