refactor: expose context window

[phanen]

Problem

To attach treesitter-context on a float window, we need to create context with a larger zindex than the float window.

However there's no way to properly achieve it, since all context win use the same config.zindex.
(so larger config.zindex cause float window overlap with the context attached on non-float window)

Solution

Make render.open accept an win_config. (But I'm not sure if this will increase maintain burden here).

Another way is expose context_winid, so we can fix zindex later by nvim_win_set_config.

(A downstream fix ibhagwan/fzf-lua#2023 via this)