Problem: `trigger` is a custom word not yet used in APIs. 

Solution: Use `get` instead because the main effect is that the 
completion candidates will be collected (and shown by default,
but follow-up commits are planned to add an `on_result` callback
that allows more general handling).

---------

Co-authored-by: Christian Clason <c.clason@uni-graz.at>

Problem: ctx is passed directly to M.trigger. In fact, it is a field of opts.

Solution: wrapped in a table and passed to M.trigger.

Problem:
vim.lsp.completion with "autotrigger" enabled, does not send
completion context, even though it has all the necessary info.

Solution:
Include the context for "autotrigger".
trigger() also optionally accepts context when manually invoked.

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`

`command` was already resolved via a `completionItem/resolve` request
but only if `additionalTextEdits` were also present, and the
`resolveSupport` capability wasn't listed.

Closes https://github.com/neovim/neovim/issues/32406