feat(lsp): run handler in coroutine to support async response (#21026)

To illustrate a use-case this also changes `window/showMessageRequest`
to use `vim.ui.select`

runtime/lua/vim/lsp/protocol.lua

@@ -20,6 +20,14 @@ function transform_schema_to_table()
 end
 --]=]
 
+---@class lsp.ShowMessageRequestParams
+---@field type lsp.MessageType
+---@field message string
+---@field actions nil|lsp.MessageActionItem[]
+
+---@class lsp.MessageActionItem
+---@field title string
+
 local constants = {
   DiagnosticSeverity = {
     -- Reports an error.
@@ -39,6 +47,7 @@ local constants = {
     Deprecated = 2,
   },
 
+  ---@enum lsp.MessageType
   MessageType = {
     -- An error message.
     Error = 1,