refactor: eliminate postion/range/selection calculation logic

Using getregionpos() (available since Nvim 0.11), may eliminate some code in vscode-neovim.

Other things such as the lsp_range() in util.lua, can probably be avoided by using vim.pos / vim.range (availabe since Nvim 0.12).

https://github.com/vscode-neovim/vscode-neovim/pull/2683#issuecomment-5742407968