Eglot: implement additionalPropertiesSupport for showMessage
This feature was introduced in LSP v3.16.
https://microsoft.github.io/language-server-protocol/specifications/lsp/3.18/specification/#window_showMessageRequest
* lisp/progmodes/eglot.el (eglot-client-capabilities): Set
window/showMessage/messageActionItem/additionalPropertiesSupport to t.
(eglot-handle-request window/showMessageRequest): Return the whole
selected MessageActionItem, not just its title.
Co-authored-by: João Távora <joaotavora@gmail.com>
(cherry picked from commit
18c81b76bc4c0fcd3effac3114763bae887a9d67)