Problem:
After https://github.com/neovim/neovim/pull/32377 selecting snippets
provided by luals inserted the multi-line text before accepting the
candidates. That's inconsistent with servers who provide `textEdit`
instead of `insertText` and having lines shift up/down while cycling
through the completion candidates is a bit irritating.
Solution:
Use the logic used for `textEdit` snippets also for `insertText`
8d7eb03040