feat: let users override icons

[hasecilu]

Previously if you wanted to change one icon across several
extensions/filenames you needed to manually override each
instance, now you can target specific icons to simplify
config file.

Basically I'm implementing this feature to match icons to
eza and lazygit, and simplify setup to avoid big config file.

config file used

Before	After

config file used

return {
  "nvim-tree/nvim-web-devicons",
  event = "VimEnter",
  opts = {
    override_by_icon = {
      [""] = { icon = "" },
      [""] = { icon = "" },
    },
  },
}

Probably it's also a good idea to:

• override color

[alex-courtis]

That's rather nice... we should have done this years ago.

My initial thought was dark/light variants, but that's a bigger task #540