start work on menus

[Andriamanitra]

TODO:

• Use menus for window/showMessageRequest
• Use menus for showReferenceLocations
• Use menus for showSymbolLocations
• Use menus for textDocument/codeAction
• namespace for menu.lua
• try to make the code less ugly (especially the event handlers)

[Andriamanitra]

Not sure if the current API is what I want to go with – I really liked the simplicity of Menu{ ["label"] = function () ... end } that I had initially. I had to make it more complicated to make the following possible:

• a header that explains what the menu is about
• multiple different actions on the same line (jump to reference with Enter or just show it with Tab)
• empty lines as separators

Maybe there's a simpler design that still allows these?

[Andriamanitra]

recording of the code actions in action: https://asciinema.org/a/706619

[Andriamanitra]

With the latest commit Rust-analyzer's code actions work too, so we can finally fill in missing trait members https://asciinema.org/a/706639 🎉