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/awk.vim

@@ -37,8 +37,8 @@ if exists("g:awk_is_gawk")
     let b:undo_ftplugin .= " | setl fp<"
   endif
 
-  " Disabled by default for security reasons.  
-  if get(g:, 'awk_exec', get(g:, 'plugin_exec', 0))
+  " Disabled by default for security reasons.
+  if dist#vim#IsSafeExecutable('awk', 'gawk')
     let path = system("gawk 'BEGIN { printf ENVIRON[\"AWKPATH\"] }'")
     let path = substitute(path, '^\.\=:\|:\.\=$\|:\.\=:', ',,', 'g') " POSIX cwd
     let path = substitute(path, ':', ',', 'g')