Add initialColor to AlphaSlider

[yangsooplus]

🎯 Goal

I have applied the initialColor to AlphaSlider, just like HsvColorPicker and BrightnessSlider, allowing you to set the initial alpha value.

🛠 Implementation details

I have implemented initialColor parameter and isInitialized flag in the same format as HsvColorPicker and BrightnessSlider.

Additionally, I have updated a portion of the initialColor description in BrightnessSlider:
it will be selected on center if you give null value.
➡️ it will be selected on the rightmost of the slider if you give a null value.

✍️ Explain examples

You can initialize the Color by passing it as the initialColor parameter, just like in HsvColorPicker and BrightnessSlider.

 AlphaSlider(
            modifier = Modifier
                .fillMaxWidth()
                .padding(10.dp)
                .height(35.dp)
                .align(Alignment.CenterHorizontally),
            controller = controller,
            initialColor = Color.Transparent,
        )

---

In the process of repeatedly selecting colors through a dialog that includes AlphaSlider, I have noticed that the alpha value resets, causing inconvenience. I would like to propose a modification to address this issue. I would like to express my gratitude for using your cool library in my personal project. 👍

[skydoves]

Looks great to me. Thank you for your contribution!

[skydoves]

Would you run the ./gradlew apiDump command before merging this PR?

[yangsooplus]

Sorry 😅 Now I run that command