Add highlight for WinSeparator

[fingolfin19]

Nvim is deprecating VertSplit and moving to WinSeparator for borders in 0.10. By default, WinSeparator links to normal which does not look great for most colorschemes.

[mngshm]

ohh, could you please share some commit hash or reference for this change of theirs ? I'll check on how to fix this issue in everblush as well.

[fingolfin19]

ohh, could you please share some commit hash or reference for this change of theirs ? I'll check on how to fix this issue in everblush as well.

I think this is the relevant commit.

Also the breaking changes doc.