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

After PR #8226 an unmapped META key in insert mode behaves like
ESC-<key> (:help i_META).

The behaviour does not fully match, since if <Esc>-<key> is pressed
manually then since it were pressed manually `gotchars` would be called
on the second <key> after insert-mode had already been left.

This would mean that `may_sync_undo` (called from `gotchars`) would
call `u_sync(FALSE)` on the second key (since we would be in normal
mode).

This overall means that <Meta-[something]> behaves differently with
respect to undo than <Esc>[something] when the [something] makes a
change.

As an example, under `nvim -u NONE`:
ihello<M-.>u

leaves the buffer empty, while
ihello<Esc>.u

leaves the buffer with one instance of `hello`.

- Fix by calling u_sync() manually in the new clause under
  `normalchar:` in `insert_handle_key`.
- Update test in tui_spec.lua that accidentally relied on the old behaviour.

closes #2454
closes #8213
ref #7972