Make highlights look better

[Chillee]

There's a couple ideas we can do for this.

1. I think we should try our best to use whatever highlights Vim provides, possibly? We can set a bunch of highlight groups :help highlight to colors provided by VSCode, but I think the choice of which color group to use should be done by vim.
2. We should coalesce each decorator on different characters into one bigger decoration.
3. Taking a look at this, there's 8 types of highlights, foreground, background, reverse, special, italic, bold, underline, and undercurl. I think we can ignore italic, bold, underline, undercurl, and special (the color used for underline/undercurl). Perhaps we can capture this very far in the future if somebody wants to use nvim linting or something dumb, but for now we'll ignore it.

As for the last 3, reverse, foreground, and background, I'm not really sure. I think we should definitely do something involving each of these highlights, but not sure what. Getting this to work would allow us to get stuff to work like Easymotion!

[Chillee]

Here's some more ideas on how to improve highlights.

1. Get specific highlight group colors. There's a nvim rpc call for this. Then, for example, we can make sure only to get highlights for searching. We may actually want to set the highlight groups ourselves to some weird color, so we know that the highlights we're seeing are for the highlight group we're looking for.
2. Don't store the screen as a 2d array. Perhaps a sparse representation of highlights might be better.