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:    VAR_SPECIAL is also used for booleans.
Solution:   Add VAR_BOOL for better type checking.
https://github.com/vim/vim/commit/9b4a15d5dba354d2e1e02871470bad103f34769a

I failed to deduce why analyzer thinks E882 may not be triggered, though 
conditions for triggering it are strange: it would trigger E882 only in the 
single case “function returned non-number”. Cases “function thrown exception”, 
or “built-in sorter encountered error” will neither yield E882 nor stop 
sort()/uniq().

Note though that searching test code revealed that neither E702 nor E882 are not 
tested anywhere.

Additionally

- Rename former tv_get_float to tv_get_float_chk due to name conflict (former
  get_tv_float is better suited for being tv_get_float).
- Add E907 error to get_tv_float() and test that it is being raised when
  appropriate.