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

Signed-off-by: D. Ben Knoble <ben.knoble+github@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/ftplugin/zig.vim

@@ -41,16 +41,13 @@ let &l:define='\v(<fn>|<const>|<var>|^\s*\#\s*define)'
 
 " Safety check: don't execute zig from current directory
 if !exists('g:zig_std_dir') && exists('*json_decode') &&
-    \  executable('zig') && get(g:, 'zig_exec', get(g:, 'plugin_exec', 0))
-    \ && (fnamemodify(exepath("zig"), ":p:h") != s:tmp_cwd
-    \ || (index(split($PATH,has("win32")? ';' : ':'), s:tmp_cwd) != -1 && s:tmp_cwd != '.'))
+    \  executable('zig') && dist#vim#IsSafeExecutable('zig', 'zig')
     silent let s:env = system('zig env')
     if v:shell_error == 0
         let g:zig_std_dir = json_decode(s:env)['std_dir']
     endif
     unlet! s:env
 endif
-unlet! s:tmp_cwd
 
 if exists('g:zig_std_dir')
     let &l:path = g:zig_std_dir . ',' . &l:path