which is both unexpected and different from the Vim behaviour.

Indent was triggered once by the '!' check in insert_execute(), and
inserting the char was correctly skipped, but then triggered again in
insert_check() (provided that cindent was not being ignored after manual
indentation, i.e. `can_cindent == true`).

While this is the smallest fix, another solution would be to remove
VimState#check and instead move that to *_enter()/-_execute(), since the
control flow is pretty unnecessarily convoluted as is. That would also
have the benefit of differing less from the Vim source code.

Neovim already removed `evim` (or any similar flags). The 'insertmode'
option is a weird remnant, so get rid of it.

The 'insertmode' option is replaced with a script that closely emulates
the option. This script is documented at :help 'insertmode'

This marks the following Vim patches as ported:

vim-patch:8.1.2333: with modifyOtherKeys CTRL-^ doesn't work

Problem:    With modifyOtherKeys CTRL-^ doesn't work.
Solution:   Handle the exception.
https://github.com/vim/vim/commit/828ffd596394f714270a01a55fc3f949a8bd9b35

vim-patch:8.1.2350: other text for CTRL-V in Insert mode with modifyOtherKeys

Problem:    Other text for CTRL-V in Insert mode with modifyOtherKeys.
Solution:   Convert the Escape sequence back to key as if modifyOtherKeys is
            not set, and use CTRL-SHIFT-V to get the Escape sequence itself.
            (closes vim/vim#5254)
https://github.com/vim/vim/commit/fc4ea2a72d36de1196a3ce17352e72f8fe90f4bb

vim-patch:8.2.2084: CTRL-V U doesn't work to enter a Unicode character

Problem:    CTRL-V U doesn't work to enter a Unicode character when
            modifyOtherKeys is effective. (Ken Takata)
Solution:   Add a flag to get_literal() for the shift key. (closes vim/vim#7413)
https://github.com/vim/vim/commit/0684e36a7ee0743f2889698fb8e0e14f7acae423

Omit getcmdkeycmd() change as it depends on Vim patch 8.2.2062, which
may introduce a potential breakage.

I'm also gonna move some other K_EVENT-related tests to vim_spec.lua

Problem:
Subdirectories like "visual", "insert", "normal" encourage people to
separate *related* tests for no good reason.  Typically the _mode_ is
not the relevant topic of a test (and when it is, _then_ create
an appropriate describe() or it()).

Solution:
- Delete the various `test/functional/<mode>/` subdirectories, move
  their tests to more meaningful topics.
- Rename `…/normal/` to `…/editor/`.
  - Move or merge `…/visual/*` and `…/insert/*` tests into here where
    appropriate.
- Rename `…/eval/` to `…/vimscript/`.
  - Move `…/viml/*` into here also.

* test(reorg): insert/* => editor/mode_insert_spec.lua
* test(reorg): cmdline/* => editor/mode_cmdline_spec.lua
* test(reorg): eval core tests => eval_spec.lua