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

When using Goneovim, Select mode `CTRL-O` returns back to Select mode
immediately (even with `--clean`). Neovim TUI (with some plugins) also randomly
returns to Select mode even if no keys are pressed when using `CTRL-O` in Select
mode.

In #8226 <A-x> and <M-x> were changed to behave like <Esc>x in insert
mode when no mapping exists. This commit backs out that change and
replaces it with a more general one that makes unmapped ALT and META
keypresses as <Esc>+char in all modes. This fixes an unnecessary and
confusing inconsistency between modes.