Change of hl for LspReference

[delafthi]

The current hl colors of

• LspReferenceText
• LspReferenceRead
• LspReferenceWrite

are very hard to see. Here a short example:

• Current settings (variables which get a value are sligtly brighter):

• With LspReferenceRead bg set to purple and LspReferenceWrite bg set to orange:

I think we do not need to set these hl groups to such dominant colors. However, more visible defaults would be nice. If you want I can make a PR with a suggestion.

These hl groups are used for vim.buf.lsp.document_highlight if the lsp supports it. How to activate this feature when resting with the cursor over a variable is described here.

[andersevenrud]

If you want I can make a PR with a suggestion.

Please do! :angel: