feat: Add nvim-treesitter-context integration

[mslalith]

Closes #264

[nullchilly]

Please use the highlight groups mentioned here https://github.com/nvim-treesitter/nvim-treesitter-context#appearance instead of the fallback group

Also use the global cnf instead of defining a local config to access the configuration

[mslalith]

@nullchilly updated. Please have a look

[nullchilly]

If too many treesitter plugins pop up I guess we should merge them into one table, like this for example:

treesitter = {
  enabled = true,
  context = true,
  rainbow = true
}