json: add specific settings for JSON

- Creates the `after/ftplugin` folders.
- Sets specific settings for JSON for folding method.

Co-Authored-by: iGor milhit <igor@milhit.ch>

after/ftplugin/json.lua

@@ -0,0 +1,8 @@
+-- Specific settings for JSON files
+--   Use treesitter for folding
+--   Disable the color column
+
+vim.opt.foldmethod="expr"
+vim.opt.foldexpr="nvim_treesitter#foldexpr()"
+vim.opt.foldlevel=3
+vim.opt.colorcolumn=""