Add support for nvim-treesitter/nvim-treesitter (nordtheme#253)

Neovim version 0.5 [1] is a long-time awaited update that introduces
features like tree-sitter [2][3] support and native LSP [4][5].
Even though Neovim devides more and more from Vim through specific
features like first-class Lua support with custom APIs, the highlighting
for tree-sitter is achived through "normal" syntax highlighting groups.
Most of the groups are already linked by the nvim-treesitter plugin by
default [6] so only a few groups have been adjuated for now to fit the
Nord style.

[1]: https://github.com/neovim/neovim/releases/tag/v0.5.0
[2]: https://github.com/tree-sitter/tree-sitter
[3]: https://github.com/nvim-treesitter/nvim-treesitter
[4]: https://neovim.io/doc/user/lsp.html
[5]: https://github.com/neovim/nvim-lspconfig
[6]: https://github.com/nvim-treesitter/nvim-treesitter/blob/90f15d9/plugin/nvim-treesitter.vim

Co-authored-by: Ferran Jovell <[email protected]>
Co-authored-by: Arctic Ice Studio <[email protected]>
Co-authored-by: Sven Greb <[email protected]>

Closes nordthemeGH-235

colors/nord.vim

@@ -277,6 +277,7 @@ call s:hi("Comment", s:nord3_gui_bright, "", s:nord3_term, "", s:italicize_comme
 call s:hi("Conceal", "", "NONE", "", "NONE", "", "")
 call s:hi("Conditional", s:nord9_gui, "", s:nord9_term, "", "", "")
 call s:hi("Constant", s:nord4_gui, "", "NONE", "", "", "")
+call s:hi("Decorator", s:nord12_gui, "", s:nord12_term, "", "", "")
 call s:hi("Define", s:nord9_gui, "", s:nord9_term, "", "", "")
 call s:hi("Delimiter", s:nord6_gui, "", s:nord6_term, "", "", "")
 call s:hi("Exception", s:nord9_gui, "", s:nord9_term, "", "", "")
@@ -301,8 +302,10 @@ call s:hi("Tag", s:nord4_gui, "", "", "", "", "")
 call s:hi("Todo", s:nord13_gui, "NONE", s:nord13_term, "NONE", "", "")
 call s:hi("Type", s:nord9_gui, "", s:nord9_term, "", "NONE", "")
 call s:hi("Typedef", s:nord9_gui, "", s:nord9_term, "", "", "")
+hi! link Annotation Decorator
 hi! link Macro Define
 hi! link PreCondit PreProc
+hi! link Variable Identifier
 
 "+-----------+
 "+ Languages +
@@ -687,6 +690,25 @@ hi! link jsNoise Delimiter
 hi! link jsPrototype Keyword
 hi! link jsRegexpString SpecialChar
 
+" tree-sitter
+" > nvim-treesitter/nvim-treesitter
+if has("nvim")
+  hi! link TSAnnotation Annotation
+  hi! link TSConstBuiltin Constant
+  hi! link TSConstructor Function
+  hi! link TSEmphasis Italic
+  hi! link TSError Error
+  hi! link TSFuncBuiltin Function
+  hi! link TSFuncMacro Function
+  hi! link TSStringRegex SpecialChar
+  hi! link TSStrong Bold
+  hi! link TSStructure Structure
+  hi! link TSTagDelimiter TSTag
+  hi! link TSUnderline Underline
+  hi! link TSVariable Variable
+  hi! link TSVariableBuiltin Keyword
+endif
+
 " TypeScript
 " > HerringtonDarkholme/yats.vim
 call s:hi("typescriptBOMWindowMethod", s:nord8_gui, "", s:nord8_term, "", s:italic, "")