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/autoload/dist/vim.vim

@@ -0,0 +1,17 @@
+vim9script
+
+# Vim runtime support library
+#
+# Maintainer:	The Vim Project <https://github.com/vim/vim>
+# Last Change:	2023 Oct 25
+
+export def IsSafeExecutable(filetype: string, executable: string): bool
+  var cwd = getcwd()
+  return get(g:, filetype .. '_exec', get(g:, 'plugin_exec', 0))
+    && (fnamemodify(exepath(executable), ':p:h') !=# cwd
+        || (split($PATH, has('win32') ? ';' : ':')->index(cwd) != -1
+            && cwd != '.'))
+enddef
+
+# Uncomment this line to check for compilation errors early
+# defcompile