Problem:
Since 2f06413dfb36 #13042, "ESC+c" sequence is treated as "ESC c"
instead of "M-c" (ALT/META+c) when not mapped, aka "fallthrough"
behavior. But "isolated" (non-ALT/META) mappings to ESC and c were not
resolved. This behavior is especially confusing for the TUI.

Solution:
Resolve isolated ESC, c mappings when there is no M-c mapping.
Change ins_char_typebuf() to escape CSI, K_SPECIAL.

fixes #13086
fixes #15869

Fix was already applied in 5f144efefa66c9694123b679bab52992279a6015 #15688,
but this commit adds another dimension to the test.

Test correctly fails after reverting 5f144efefa66c9694123b679bab52992279a6015.

ref #15288

Problem
- `redir_exec` is obsolete, but it keeps getting used in new tests
  because people copy existing tests.
- Disadvantages of `redir_exec`:
  - Captures extra junk before the actual error/message that we _want_ to test.
  - Does not fail on error, unlike e.g. `command()`.

Solution
- Use new functions like `nvim_exec` and `pcall_err`.

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