Add support for LSPSignatureHelpActiveParameter (#286)

The `vim.lsp.buf.signature_help` function is used to highlight the
active parameter in the signature help [1].
Before this commit the active parameter was not styled differently to
any other parameter which made it hard to distinguish it. This has been
improved by adding support for the `LspSignatureActiveParameter` syntax
highlighting group where the active parameter now uses `nord8` are
foreground color and additionally a font underline with the same color.

[1]: https://github.com/neovim/neovim/blob/70db972e5fbcab39946ad8ac05472a693cf65b68/runtime/doc/lsp.txt#L456-L459


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

GH-286

colors/nord.vim

@@ -231,6 +231,9 @@ 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, "", "")
 
+"+- Neovim LspSignatureHelp -+
+call s:hi("LspSignatureActiveParameter", s:nord8_gui, "", s:nord8_term, "", s:underline, "")
+
 "+--- Gutter ---+
 call s:hi("CursorColumn", "", s:nord1_gui, "NONE", s:nord1_term, "", "")
 if g:nord_cursor_line_number_background == 0