feat: rename and deprecate neovide_transparency to neovide_opacity

[lucobellic]

Rename neovide_transparency to neovide_opacity.
Changes to neovide_transparency will update WindowSettings::opacity.

• Update all documentation and references to use neovide_opacity.
• Rename WindowSettings transparency to opacity for consistency.
• Add alias attribute to the SettingGroup macro.
• Update the test test_read_initial_values with alias.
  Test removed since aliases are excluded from read_initial_values to not trigger deprecation message at startup.
• Add deprecation message for settings with alias attribute.
  Error: neovide_transparency has now been deprecated, use neovide_opacity instead.

I don't know if a deprecation message is needed and how to properly handle neovide_transparency deprecation, let me know.

What kind of change does this PR introduce?

• Feature

Did this PR introduce a breaking change?

• No

Feel free to edit or request any changes

Close #2701

[fredizzimo]

For the deprecation warning, you can model it after this:
https://github.com/neovide/neovide/blob/000b75c0f3bcfb602967cc5c2e8a9d584be06a02/src/window/macos.rs#L180C12-L180C39

[fredizzimo]

The macOS build failure is not your fault. Something has changed in the nightly compiler. This might fix it, but I'm not sure:

• chore: update objc2 and remove icrate. also some refactor. #2768

[lucobellic]

Code updated with deprecation message.
I just had to exclude transparency key from read_initial_values to avoid triggering the deprecation message at startup.