Problem: autotrigger option of vim.lsp.completion.enable() would trigger
all clients, as long as it matched at least one client's
triggerCharacters.
Solution: trigger only the clients with triggerCharacters matching the
character. overtriggering still happens if any client returns
isIncomplete=true (this case is more involved).
Co-authored-by: Mathias Fussenegger <f.mathias@zignar.net>