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

close #11459

- nvim_buf_get_extmarks: rename "amount" => "limit"
- rename `set_extmark_index_from_obj`

The "technically correct" interpretation is to execute the first line
that is seen (and this is what happens on middle-click paste in Vim).
^M is only intended to "defuse" the newline, so the user can review it.

The parent commit changed the behavior to insert <Space> between lines,
but that's a higher-risk change: it is arguably possible that some user
*wants* the literal ^M chars when e.g. assigning to a register:
    :let @a='<C-R>b'

To avoid that risk, keep the old behavior and only omit the last ^M.
This makes `yy:<C-R>0` nicer at no cost.

^M isn't any more "correct" than space: the "technically correct"
interpretation is to execute the first line that is seen (and this is
what happens on middle-click paste in Vim). ^M is only intended to
defuse the newline, so that the user can review the command. We can do
that with a space instead, and then the command can be executed without
having to fix it up first.