Extend the capabilities of is_os to detect more platforms such as
freebsd and openbsd. Also remove `iswin()` helper function as it can be
replaced by `is_os("win")`.

Problem:    The eval.txt help file is way too big.
Solution:   Move the builtin function details to a separate file.
https://github.com/vim/vim/commit/1cae5a0a034d0545360387407a7a409310f1efe2

Note: Neovim-specific references to |functions| were changed to
|builtin-functions|. This included updates to:
  1. test/functional/vimscript/functions_spec.lua
  2. test/functional/vimscript/eval_spec.lua
  3. runtime/doc/lua.txt

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