Add support for pandoc (#220)

Add support for `vim-pandoc/vim-pandoc-syntax`

To improve syntax highlighting for Pandoc [1], support for the
vim-pandoc/vim-pandoc-syntax [2] plugin has been implemented.
Most groups are linked to existing Markdown groups to ensure a
consistent style across languages and different plugins.

Configurations used for testing:

```vim
let g:pandoc#syntax#conceal#cchar_overrides = {"atx": "〉"}
let g:nord_italic = 1
```

Resources:

- `g:pandoc#syntax#conceal#cchar_overrides` docs [3]
- `s:cchars` definition [4]
- Markdown "Extended Syntax" Guide [5]

[1]: https://pandoc.org
[2]: https://github.com/vim-pandoc/vim-pandoc-syntax
[3]: https://github.com/vim-pandoc/vim-pandoc-syntax/blob/68d7249/doc/pandoc-syntax.txt#L45-L50
[4]: https://github.com/vim-pandoc/vim-pandoc-syntax/blob/5056e63/syntax/pandoc.vim#L37-L72
[5]: https://www.markdownguide.org/extended-syntax

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

Closes GH-220

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