LibJS: Replace standalone js_bigint() with BigInt::create()

Three standalone Cell creation functions remain in the JS namespace:

- js_bigint()
- js_string()
- js_symbol()

All of them are leftovers from early iterations when LibJS still took
inspiration from JSC, which itself has jsString(). Nowadays, we pretty
much exclusively use static create() functions to construct types
allocated on the JS heap, and there's no reason to not do the same for
these.
Also change the return type from BigInt* to NonnullGCPtr<BigInt> while
we're here.

This is patch 1/3, replacement of js_string() and js_symbol() follow.

Userland/Libraries/LibJS/Runtime/BigIntConstructor.cpp

@@ -80,11 +80,11 @@ JS_DEFINE_NATIVE_FUNCTION(BigIntConstructor::as_int_n)
     // NOTE: Some of the below conditionals are non-standard, but are to protect SignedBigInteger from
     //       allocating an absurd amount of memory if `bits - 1` overflows to NumericLimits<size_t>::max.
     if ((bits == 0) && (mod >= BIGINT_ONE))
-        return js_bigint(vm, mod.minus(bits_shift_left));
+        return BigInt::create(vm, mod.minus(bits_shift_left));
     if ((bits > 0) && (mod >= BIGINT_ONE.shift_left(bits - 1)))
-        return js_bigint(vm, mod.minus(bits_shift_left));
+        return BigInt::create(vm, mod.minus(bits_shift_left));
 
-    return js_bigint(vm, mod);
+    return BigInt::create(vm, mod);
 }
 
 // 21.2.2.2 BigInt.asUintN ( bits, bigint ), https://tc39.es/ecma262/#sec-bigint.asuintn
@@ -99,7 +99,7 @@ JS_DEFINE_NATIVE_FUNCTION(BigIntConstructor::as_uint_n)
     // 3. Return the BigInt value that represents ℝ(bigint) modulo 2bits.
     // FIXME: For large values of `bits`, this can likely be improved with a SignedBigInteger API to
     //        drop the most significant bits.
-    return js_bigint(vm, modulo(bigint->big_integer(), BIGINT_ONE.shift_left(bits)));
+    return BigInt::create(vm, modulo(bigint->big_integer(), BIGINT_ONE.shift_left(bits)));
 }
 
 }