Add HTML space rendering to lsp-ui-doc

[gbrlgn]

Some LSPs might end up rendering some spaces as HTML, such as what is happening in the YAML Language Server, since a commit introduced this kind of weird spacing that parses &emsp for spaces, whose are rendered literally by lsp-ui-doc as depicted below:

This happens in all instances of lsp-ui-doc, be it the hover box or the minibuffer, as these characters are coming directly from the language server responses. Ideally, they shouldn't even get rendered, as this kind of spacing would look awkward, but the fact that these characters are getting rendered as plaintext confuses and annoys me.

Some users in the Neovim community have come up with some workarounds but I don't really know if they're needed in this situation, however, I think they might also be taken as a clue for whatever is happening here.