add support for floating window and popup

It would be good if vital.vim has a popup api that works in both vim and neovim. There has been some PRs related to this in vim-lsp at https://github.com/prabirshrestha/vim-lsp/pull/510 and https://github.com/prabirshrestha/vim-lsp/pull/567. Another one i found was this https://github.com/ZSaberLv0/ZFVimPopup. Lot of other plugins also has to keep creating wrappers to solve this issue.

Besides just a wrapper around vim and neovim apis there are other high level apis that would be necessary for the popup such as controlling direction and zindex. For example I might want to show LSP hover information above the cursor but diagnostics below the cursor. If autocomplete popup menu is open I might want to show the popup at the right of the complete menu but if it is to the end of the screen i might want to show it to the left instead or if at the bottom of the screen i might want to show at the top. Or might even want to dock it similar as https://github.com/ncm2/float-preview.nvim. Might also want to listen to buffer/window resizes and resize accordingly.

Since some of these are quite complicated specially when dealing with multiple popup at the same time my thought was to have 2 modules.

1. Popup : This is a simple wrapper that is just bridges vim popup and neovim floating window apis.
2. PopupManager: This is the bigger one which does the management of multiple popups.

Start by comparing Vim's popup API with Neovim's floating-window API and review the linked vim-lsp and ZFVimPopup implementations. Define the scope of the proposed Popup and PopupManager modules, including positioning, z-index, multiple popups, and resize handling. Done should mean a documented cross-editor API with tests covering the agreed behavior.