Add support for LSP textDocument/documentHighlight (#284)

Support for Neovim LSP `documentHighlight` groups

The Neovim LSP `textDocument` / `documentHightlight` groups [1] are
responsible to highlight tokens in a document that are related to each
other, e.g. decalred variables, using the
`vim.buf.lsp.document_highlight()` function.
Also see the LSP specification about "Document Highlights Request" [2]
for more details.

[1]: https://github.com/neovim/neovim/blob/f92a2457c2e7ad14d9a5a907ef4213fa770b6d95/runtime/doc/lsp.txt#L423
[2]: https://microsoft.github.io/language-server-protocol/specification#textDocument_documentHighlight


Co-authored-by: Arctic Ice Studio <development@arcticicestudio.com>
Co-authored-by: Sven Greb <development@svengreb.de>

colors/nord.vim

@@ -226,6 +226,11 @@ call s:hi("DiagnosticUnderlineError" , s:nord11_gui, "", s:nord11_term, "", "und
 call s:hi("DiagnosticUnderlineInfo" , s:nord8_gui, "", s:nord8_term, "", "undercurl", "")
 call s:hi("DiagnosticUnderlineHint" , s:nord10_gui, "", s:nord10_term, "", "undercurl", "")
 
+"+- Neovim DocumentHighlight -+
+call s:hi("LspReferenceText", "", s:nord3_gui, "", s:nord3_term, "", "")
+call s:hi("LspReferenceRead", "", s:nord3_gui, "", s:nord3_term, "", "")
+call s:hi("LspReferenceWrite", "", s:nord3_gui, "", s:nord3_term, "", "")
+
 "+--- Gutter ---+
 call s:hi("CursorColumn", "", s:nord1_gui, "NONE", s:nord1_term, "", "")
 if g:nord_cursor_line_number_background == 0