Add set_alpha filter

[Davenchy]

Hello,

I hope you are doing well.

I added a set_alpha filter. Additionally, I implemented two new functions, format_rgba_float and format_hsla_float, to address the alpha channel value formatting. The original functions print the alpha value in the range 0 to 255, while it should be in the range 0.0 to 1.0.

I also updated the example/colors.whatever-extension file with a simple usage example of the set_alpha filter and made an entry in the changelog under the "unreleased" section to document this addition.

I noticed that the current implementation represents the alpha channel value in the range 0 to 255. As per my understanding, it should be in the range 0.0 to 1.0. I did not remove any existing code but added the filter implementation and the two methods to correct the output of the filter.

I suggest reconsidering the current implementation of the alpha channel value. I am ready to contribute further.

[Davenchy]

Here is a real example demonstrating the need for the set_alpha filter.

In my swaync CSS file, I used the set_alpha filter to set the background of the notification center to 0.7, as shown in the screenshot.

[InioX]

Thanks! I will fix the alpha value to be a float.