feat(plugin): expose colors table

[minusfive]

This PR exposes the colors table through the plugin module table, so the full predefined palette can be used to customize the theme, etc.

Usage e.g.:

local wezterm = require("wezterm")
local catppuccin = wezterm.plugin.require("https://github.com/catppuccin/wezterm")
-- ...
local ctp = wezterm.color.get_builtin_schemes()["Catppuccin Mocha"]
ctp.tab_bar.inactive_tab.fg_color = catppuccin.colors.mocha.overlay2
config.color_schemes = {
  ["Catppuccin Mocha (custom)"] = ctp,
}