use neovim core treesitter API instead of nvim-treesitter plugin

[silvercircle]

Hi,

Can you please look into this PR.

It uses Neovim's core Treesitter API instead of the nvim-treesitter plugin. The motivation behind this is that nvim-treesitter will be rewritten (see: https://github.com/nvim-treesitter/nvim-treesitter/tree/main) and this will break the outline plugin because nvim-treesitter.parsers.get_parser() will be removed.

This should work with Neovim 0.9 or later. Maybe it would make sense to wrap this around an option.

[hedyhli]

This should work with Neovim 0.9 or later. Maybe it would make sense to wrap this around an option.

An option is not needed since we can check nvim version from lua, but anyway, seems like this works with Neovim 0.7. Thanks!