Fix: properly update highlights on colorscheme change

[theofabilous]

Fixes incorrect highlight updates when colorscheme is changed. This is only relevant to this feature branch.

Prior to these changes, any "patched" devicon highlight was not "re-patched" when colorscheme is changed (by patched, I mean "create a separate highlight group for the devicon that inherits the winbar background highlights"). Now this is done.

Additionally, the ColorScheme autocmd event now schedules the updating of highlights instead of triggering it automatically. This is useful when the user sets some custom highlights after calling :colorscheme. Prior to this, if the user did something like:

vim.cmd('colorscheme <some colorscheme>')
vim.api.nvim_set_hl(0, 'WinBar', { ... })

the highlights would be created before the WinBar highlight is manually set, and thus not reflected in DropBar.
Now, since the update is scheduled, the above example would be reflected in DropBar highlights.

[theofabilous]

I suspect #69 will fix the failing tests in this PR. I created a separate PR for the fix as the changes are unrelated.

[Bekaboo]

Thanks for your contribution! Sorry for the late reply.