Attn: Joao Tavora

The attached patch adds basic support to eglot for the
window.showDocument downcall, added in LSP 3.16, which enables the
server to request that
the client open an URL either in an external browser (e.g. as if by
the open(1) or xdg-open(1) command) or internally, in the editor.
https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#window_showDocument

See also https://github.com/joaotavora/eglot/discussions/1180.

Thanks to @nemethf, whose unmerged PR
https://github.com/joaotavora/eglot/pull/855 provided numerous
improvements over my own first draft.

This patch can be applied to the base commit of 8ee205d. Please let me
know if you'd like it in some other form.

cheers
alan