Better dealing with hidden and non-interactive terminals

[jam1015]

Over the past two days issue #420, pr #421, and pr #422 dealt with how the Neovim target handles non-interactive terminals, and unlisted terminals.

But pr #421 and #422 are in conflict with each other. I aimed to fix the problems that those PRs addressed and resolve the conflict between them by creating an additional user-configurable option.

This PR:

• adds a check in SlimeAddChannel to make sure that a terminal that is opened has the &channel setting, which is what determines if it is a valid target. I did a quick test with neotest and as mentioned in issue #420 , following the example tests in this tutorial, and an error as described came up when this change wasn't added, and was fixed when it was.

• adds a configurable option , g:slime_neovim_ignore_unlisted that turns on or off unlisted buffers being available as targets. This addresses PR #422

• adds documentation of these changes

[jam1015]

to be 100% clear, I think this is ready to merge