feat: allow mixing highlight colors

[soulfresh]

What does this PR do?

This update allows users to define highlight colors that are a mix of two other colors in the theme. By specifying the mix colors as color names from the theme (ex "green" or "red"), the highlight groups will adapt to match the current theme as the user switches themes. This includes switching between light and dark themes.

hl_override = {
  DiffAdd = {
    -- Set the background of diff additions to a color
    -- 50% between the green and black values defined
    -- in the theme. For light themes, this will actually
    -- be a mix between green and white colors, resulting
    -- in a light green background. Dark themes will show
    -- a dark green background.
    bg = {"green", "black", 50}
  }
}

How do I test it?

1. Set the following in your plugin spec

  {
    "NvChad/base46",
    branch = "feat/highlight-color-mix",
    url = "https://github.com/soulfresh/base46",
    build = function()
      require("base46").load_all_highlights()
    end,
  },

2. Add the following to your hl_override spec

  DiffAdd = {
    fg = "green",
    bg = {"green", "black", 90 },
  },

3. Restart nvim, open a diff and switch themes