fix(showMessage): use on_message handler instead of _on_message

Fixes typo where M._on_message (undefined) was assigned as the LSP
handler instead of M.on_message. LSP window/showMessage was silently broken.

From upstream PR folke/noice.nvim#1103

Co-Authored-By: tris203 <tris203@users.noreply.github.com>

Author: njhoffman

lua/noice/lsp/message.lua

@@ -17,7 +17,7 @@ M.message_type = {
 ---@alias ShowMessageParams {type:MessageType, message:string}
 
 function M.setup()
-  vim.lsp.handlers["window/showMessage"] = M._on_message
+  vim.lsp.handlers["window/showMessage"] = M.on_message
 end
 
 ---@param result ShowMessageParams