runtime(dist): centralize safe executable check and add vim library (vim/vim#13413)

Follow up to 816fbcc26 (patch 9.0.1833: [security] runtime file fixes,
2023-08-31) and f7ac0ef50 (runtime: don't execute external commands when
loading ftplugins, 2023-09-06).

This puts the logic for safe executable checks in a single place, by introducing
a central vim library, so all filetypes benefit from consistency.

Notable changes:
- dist#vim because the (autoload) namespace for a new runtime support
  library. Supporting functions should get documentation. It might make
  life easier for NeoVim devs to make the documentation a new file
  rather than cram it into existing files, though we may want
  cross-references to it somewhere…
- The gzip and zip plugins need to be opted into by enabling execution
  of those programs (or the global plugin_exec). This needs
  documentation or discussion.
- This fixes a bug in the zig plugin: code setting s:tmp_cwd was removed
  in f7ac0ef50 (runtime: don't execute external commands when loading
  ftplugins, 2023-09-06), but the variable was still referenced. Since
  the new function takes care of that automatically, the variable is no
  longer needed.

https://github.com/vim/vim/commit/cd8a3eaf5348feacfecab4b374b7ea4ce6a97422

Co-authored-by: D. Ben Knoble <ben.knoble+github@gmail.com>

runtime: don't execute external commands when loading ftplugins

This is a followup to 816fbcc262687b81fc46f82f7bbeb1453addfe0c (patch
9.0.1833: [security] runtime file fixes)

It basically disables that external commands are run on loading of the
filetype plugin, **unless** the user has set the `g:plugin_exec = 1`
global variable in their configuration or for a specific filetype the
variable g:<filetype>_exec=1.

There are a few more plugins, that may execute system commands like
debchangelog, gitcommit, sh, racket, zsh, ps1 but those do at least
do not run those commands by default during loading of the filetype plugin
(there the command is mostly run as convenience for auto-completion or
to provide documentation lookup).

closes: vim/vim#13034

https://github.com/vim/vim/commit/f7ac0ef5098856bedca26e7073594a407c05636f

Co-authored-by: Christian Brabandt <cb@256bit.org>
Co-authored-by: Tim Pope <vim@tpope.org>

Update runtime files
https://github.com/vim/vim/commit/3132cddd209ee510bde48b6520290cb26c8f604a

Omit :sort,sort() changes.

Update runtime files.
https://github.com/vim/vim/commit/d58a3bf7dac8d53faf42e13cc1152b110f12c404

Omit syntax/man.vim.

Updated runtime files.

https://github.com/vim/vim/commit/3df0173fa6d0418e89ef4e9c1d04a97c92eec27c