Add type annotations to improve development experience

[jakubbortlik]

Feature Description

When working on the plugin, I'm getting a lot of warnings about undefined types, e.g., NuiTree or NuiTree.Node: @harrisoncramer , do you also get these warnings or I am just missing something? The many warnings make it easier to miss some more relevant problems with the code. In #299, I've added an annotation for RootNode (lua/gitlab/actions/discussions/annotations.lua:137), precisely to get rid of some irrelevant warnings. If there is no better way, I'd suggest to add annotations also for a regular Node and for NuiTree. Or maybe this should be done in the nui.nvim library, I don't know.