Add support for Nvim LSP client

[svilenb]

This PR adds highlighting for the build-in Nvim LSP client.

Things that may still need to be added in order to complete the support for that client are the addition of the following highlight groups:

                                                           *hl-LspReferenceText*
LspReferenceText          used for highlighting "text" references
                                                           *hl-LspReferenceRead*
LspReferenceRead          used for highlighting "read" references
                                                          *hl-LspReferenceWrite*
LspReferenceWrite         used for highlighting "write" references

[ellisonleao]

any updates on this ?

[rbong]

@ellisonleao it's merged in the gruvbox-community/gruvbox fork if you need it