chore: complete the diagnostic setup migration

Colors groups and icons are now in diagnostics.lua. They are defined on
setup which allows an easier configuration and better documentation.
`lsp_diagnostics` boolean value has been moved into a table `diagnostics`
with `enable` and `icons` as properties.

lua/nvim-tree/colors.lua

@@ -70,10 +70,6 @@ local function get_links()
     FileDeleted = 'NvimTreeGitDeleted',
     Popup = 'Normal',
     GitIgnored = 'Comment',
-    LspDiagnosticsError = "LspDiagnosticsDefaultError",
-    LspDiagnosticsWarning = "LspDiagnosticsDefaultWarning",
-    LspDiagnosticsInformation = "LspDiagnosticsDefaultInformation",
-    LspDiagnosticsHint = "LspDiagnosticsDefaultHint",
     StatusLine = "StatusLine",
     StatusLineNC = "StatusLineNC",
     SignColumn = 'Normal',