Problem:
User cannot configure the tool used by `vim.ui.open` (or `gx`). With
netrw this was supported by `g:netrw_browsex_viewer`.
Solution:
Introduce `opts.cmd`. Users that want to set this globally can
monkey-patch `vim.ui.open` in the same way described at `:help vim.paste()`.
Fixes https://github.com/neovim/neovim/issues/29488
Co-authored-by: Justin M. Keyes <justinkz@gmail.com>