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/perl.vim

@@ -56,12 +56,8 @@ endif
 
 " Set this once, globally.
 if !exists("perlpath")
-    let s:tmp_cwd = getcwd()
     " safety check: don't execute perl binary by default
-    if executable("perl") && get(g:, 'perl_exec', get(g:, 'plugin_exec', 0))
-        \ && (fnamemodify(exepath("perl"), ":p:h") != s:tmp_cwd
-        \ || (index(split($PATH, has("win32") ? ';' : ':'), s:tmp_cwd) != -1
-        \ && s:tmp_cwd != '.'))
+    if dist#vim#IsSafeExecutable('perl', 'perl')
       try
 	if &shellxquote != '"'
 	    let perlpath = system('perl -e "print join(q/,/,@INC)"')
@@ -77,7 +73,6 @@ if !exists("perlpath")
 	" current directory and the directory of the current file.
 	let perlpath = ".,,"
     endif
-    unlet! s:tmp_cwd
 endif
 
 " Append perlpath to the existing path value, if it is set.  Since we don't