plugins/alpha: migrate to mkNeovimPlugin

[HeitorAugustoLN]

No description provided.

[MattSturgeon]

I'm a little confused. You have a settings option, but you are also manually calling setup and passing it a theme option.

The idea of settings is that it is the nix representation of the lua passed to setup.

Usually, mkNeovimPlugin will take responsibility for calling setup and passing it the settings.

[HeitorAugustoLN]

.setup needs to be manually called when using theme option. Because that is the only way we can set pre-defined themes. That's why, theme and settings.layout options can't be used at the same time

[MattSturgeon]

If so, that also implies:

1. settings as a whole must not be used when a theme is defined
2. setup must always be called manually, either passing it settings or theme

[MattSturgeon]

Interesting, the upstream examples show a different usage:

require('alpha').setup(require('alpha.themes.dashboard').config)

They are passing pre-defined settings to setup; getting those settings from a theme module.

[HeitorAugustoLN]

It is the same usage, the theme option is being converted here:

nixvim/plugins/by-name/alpha/default.nix

Lines 196 to 201 in c83415c

	apply =
	value:
	if builtins.isString value then
	lib.nixvim.mkRaw ''require("alpha.themes.${value}").config''
	else
	value;