runtime(tar): fix a few problems with the tar plugin

From: #138331:
  - Updating .tar.zst files was broken. Fixes #12639.
  - Extracting files from .tar.zst / .tzs files was also broken and
    works now.
From: #12637:
  - Fixes variable assignment and typo
From: #8109:
  - Rename .tzs to the more standard .tzst

fixes: #12639
fixes: #8105
closes: #8109
closes: #12637
closes: #13831

Co-authored-by: Martin Rys <martin@rys.pw>
Co-authored-by: Eisuke Kawashima <e-kwsm@users.noreply.github.com>
Co-authored-by: Carlo Teubner <carlo@cteubner.net>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/runtime/plugin/tarPlugin.vim b/runtime/plugin/tarPlugin.vim
index d55492a..40d2242 100644
--- a/runtime/plugin/tarPlugin.vim
+++ b/runtime/plugin/tarPlugin.vim
@@ -45,7 +45,7 @@
   au BufReadCmd   *.tar.xz		call tar#Browse(expand("<amatch>"))
   au BufReadCmd   *.txz			call tar#Browse(expand("<amatch>"))
   au BufReadCmd   *.tar.zst		call tar#Browse(expand("<amatch>"))
-  au BufReadCmd   *.tzs			call tar#Browse(expand("<amatch>"))
+  au BufReadCmd   *.tzst			call tar#Browse(expand("<amatch>"))
 augroup END
 com! -nargs=? -complete=file Vimuntar call tar#Vimuntar(<q-args>)