fix(ghost-text): flickering using decoration provider and redraw func…

[giuxtaposition]

Should fix #249.
I'm basically using vim.api.nvim_set_decoration_provider and the redraw function to update the ghost_text

[Saghen]

Thanks for the PR! I've switched it from a decoration provider to autocmds. I wanted to avoid redrawing the window since that can sometimes be expensive and setting non-ephemeral extmarks has some weird behaviors in a decoration provider.