this works similar to <c-o> or <c-\><c-o> in insert mode

It just types :terminal<CR> into the terminal, which is redundant.

Prevent on_lines emitting out-of-bounds line indexes.

Problem:    Cannot distinguish Normal and Terminal-Normal mode.
Solution:   Make mode() return "nt" for Terminal-Normal mode. (issue vim/vim#8856)
https://github.com/vim/vim/commit/72406a4bd2896915b6f541e26d41521a59b1f846

When entering terminal mode, cursorlineopt is no longer entirely
disabled. Instead, it's set to `number`. Doing so ensures that users
using `set cursorline` combined with `set cursorlineopt=number` have
consistent highlighting of the line numbers, instead of this being
disabled when entering terminal mode.

Co-authored-by: Gregory Anders <greg@gpanders.com>
Co-authored-by: Sean Dewar <seandewar@users.noreply.github.com>

Problem:
jobwait() returns early if the job was stopped, but the job might have
pending callbacks on its event queue which are required to complete its
teardown. State such as term->closed might not be updated yet (by the
pending callbacks), so codepaths such as :bdelete think the job is still
running.

Solution:
Always flush the job's event queue before returning from jobwait().

ref #15349