switch to calver; add g:vimwiki_version; tidy version hardcoding

Setting a global for g:vimwiki_version doesn't seem too egregious, and
lets us reference it cleanly elsewhere.

Removes version header from vimwiki.txt and hardcoding in tagfile
generation.

There's still one instance of that left in test/tag.vader that we should
clean up eventually.

plugin/vimwiki.vim

@@ -11,7 +11,7 @@ endif
 let g:loaded_vimwiki = 1
 
 " Set to version number for release:
-let s:plugin_vers = '2.6.0'
+let g:vimwiki_version = '2022.12.02'
 
 " Get the directory the script is installed in
 let s:plugin_dir = expand('<sfile>:p:h:h')
@@ -209,7 +209,7 @@ endfunction
 " Echo vimwiki version
 " Called by :VimwikiShowVersion
 function! s:get_version() abort
-  echo 'Version: ' . s:plugin_vers
+  echo 'Version: ' . g:vimwiki_version
   let l:plugin_rev    = system('git --git-dir ' . s:plugin_dir . '/.git rev-parse --short HEAD')
   let l:plugin_branch = system('git --git-dir ' . s:plugin_dir . '/.git rev-parse --abbrev-ref HEAD')
   let l:plugin_date   = system('git --git-dir ' . s:plugin_dir . '/.git show -s --format=%ci')