Conceal highlighting group support (nordtheme#261)

The `Conceal` group was not supported which could have led to rendering
problems for (Unicode) characters that require special encoding like
the ones from the Greek alphabet [1] that are often used in LaTeX [3] or
Pandoc [4] documents. These characters were highlighted with the default
background color which makes them kind of unreadable with the theme
foreground color.
See `:help conceal` and `:help concealcursor` for more details about
concealing in Vim.

To fix the problem, the `Conceal` group has been added with its
background color set to `NONE` for GUI and terminal mode to either use
the terminal default background color or let loaded scripts apply custom
styles based on the current runtime context.

The problem has been reported in nordthemeGH-149, nordthemeGH-207 and nordthemeGH-211 with LaTeX
and Pandoc being used by the reporters. PR nordthemeGH-220 adds support for
vim-pandoc/vim-pandoc-syntax [5] specific highlighting groups and can be
merged after adding basic support for `Conceal`.

[1]: https://en.wikipedia.org/wiki/Greek_alphabet
[2]: https://www.overleaf.com/learn/latex/mathematical_expressions
[3]: https://www.latex-project.org
[4]: https://pandoc.org
[5]: https://github.com/vim-pandoc/vim-pandoc-syntax

Co-authored-by: Sven Greb <[email protected]>

Closes nordthemeGH-256

colors/nord.vim

@@ -274,6 +274,7 @@ endif
 call s:hi("Boolean", s:nord9_gui, "", s:nord9_term, "", "", "")
 call s:hi("Character", s:nord14_gui, "", s:nord14_term, "", "", "")
 call s:hi("Comment", s:nord3_gui_bright, "", s:nord3_term, "", s:italicize_comments, "")
+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("Define", s:nord9_gui, "", s:nord9_term, "", "", "")