Link all default highlight groups to existing ones by default

[vandalt]

Hello!

The current default highlights in define_highlights() are set to specific colors. This works well with the default theme in dark mode, but not so much in light mode. The same may be true for other themes that do not define the Neotest* highlight groups. The Cyan, Yellow, and Green colors (or their hex counterparts) were especially difficult to see for me (I'm colorblind, which probably makes the matter worse).

This PR simply changes the default highlights to existing highlight groups, instead of specific colors.This should make the colors more consistent and accessible for most colorschemes. It does for the default one, which I have tested. I tried to keep the new colors as close as possible to the old ones. An alternative fix would be for users to handle this in the highlights config, but this would override the groups for colorschemes that do set the Neotest* groups.

The second commit changes the vim.cmd to separete vim.api.nvim_set_hl calls. Let me know if you would prefer that I revert this.

Thank you!

Fixes #310