`nvim_(get|set)_option_value` pick the current buffer / window by default for buffer-local/window-local (but not global-local) options. So specifying `buf = 0` or `win = 0` in opts is unnecessary for those options. This PR removes those to reduce code clutter.

Co-authored-by: zeertzjq <zeertzjq@outlook.com>
Co-authored-by: famiu <famiuhaque@protonmail.com>

test: replace lfs with luv

luv already pretty much does everything lfs does, so this duplication
of dependencies isn't needed.

Problem:    No error when assigning bool to a string option with setwinvar().
Solution:   Give an error (closes vim/vim#10766)
https://github.com/vim/vim/commit/28f84e17b068daca2635692d279930dcb7a150d0

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