Support alpha to hex

Generally, hex color is used as 3 or 6 digits, but it can be used by adding 2-digits meaning alpha.
https://developer.mozilla.org/en-US/docs/Web/CSS/hex-color


current:
```ts
new Color('4aa444').setAlhap(0.5).toHex()
```
returns the same value as 4aa444.

I would like to add a method like toHex8() to returnvalue that includes the alpha value.