Full HTML widget support in a GUI

There are a number of neovim-based GUIs.

Some notable ones are:

[uivonim](https://github.com/smolck/uivonim)
[vimr](https://github.com/qvacua/vimr)
[gnvim](https://github.com/vhakulinen/gnvim)
[vv](https://github.com/vv-vim/vv)

and likely more, given I don't personally stay too much on top of these. The first one is likely particularly interesting given it has contributors who are neovim core devs.

We currently support TUI-based widgets, but for more complex ones such as e.g. the [sudoku solver](https://leanprover-community.github.io/lt2021/slides/EWAyers-Widgets-LT2021.pdf) which truly rely on HTML rendering, a GUI (with access to a real browser) is needed.

We could attempt to follow something like [bracey's model](https://github.com/turbio/bracey.vim) even for terminal nvim by opening an optional browser window which updates not on save but on infoview update.