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

Problem:    Cannot recognize a <script> mapping using maparg().
Solution:   Add the "script" key. (closes vim/vim#5873)
https://github.com/vim/vim/commit/2da0f0c445da3c9b35b2a0cd595d10e81ad2a6f9

This is a refactoring typo from #6947.

Fixes #6985

Combined with #6947 where typo was made it also fixes vim/vim#1827 which was
present in Neovim.

* Add api function get keymap

nvim_get_keymap(mode)
nvim_buf_get_keymap(buffer, mode)