## Context

GPUI API change only: 
  - added `.alpha(a: f32)` method to `RGBA`
  - added `.opacity(factor: f32)` method to `RGBA`
  - added documentation for both new methods
  - added test coverage for both methods
  
These implementations already exist for the `HSLA` struct and were
missing for `RGBA`.

> [!NOTE]
> In my opinion, a trait implementation could be beneficial here. Let me
know what you think about that idea before I end up making unwanted
changes.

## How to Review

Just added the new methods, including documentation and test coverage.

## Self-Review Checklist

<!-- Check before requesting review: -->
- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Release Notes:

- Added `gpui::Rgba::alpha` and `gpui::Rgba::opacity` methods for
setting and scaling a color's alpha channel, matching the existing
`gpui::Hsla` API

---------

Co-authored-by: dino <dinojoaocosta@gmail.com>