Syntax: Fasten highlighting, from 200ms to 20ms for first screen update

Problem:

The loading of a .md file was slow due to multiple nested lookback.
It is advised in vim help to prefer \zs, see :h /\@<
Anyway, it is useless to nest them.

089.080  000.085  000.085: sourcing /home/mtourneb/.vim/after/syntax/vimwiki.vim
089.264  005.184: opening buffers
089.322  000.058: BufEnter autocommands
089.323  000.001: editing files in windows
089.414  000.091: VimEnter autocommands
089.415  000.001: before starting main loop
303.698  214.283: first screen update
303.700  000.002: --- VIM STARTED ---

Solution:

Remove the function s:expand_delimiter in vars.vim.
It was doing a duplicate job with vimwiki#u#hi_expand_regex
And ... well avoid lookback as much as possible

091.784  000.087  000.087: sourcing /home/mtourneb/.vim/after/syntax/vimwiki.vim
091.991  006.034: opening buffers
092.050  000.059: BufEnter autocommands
092.052  000.002: editing files in windows
092.172  000.120: VimEnter autocommands
092.173  000.001: before starting main loop
112.574  020.401: first screen update
112.578  000.004: --- VIM STARTED ---

test/syntax.vader

@@ -37,6 +37,7 @@ Given vimwiki (Markdown typeface with escape sequence #1044: _ __ * ** {{{2):
   a ~~staaill deleted~~ Del 9
   $$Eq\$$ uation follows$ Math 10
   `code \` not finished inline` Code 11
+  012345678901234567890
 
 Execute (Set syntax markdown):
   call SetSyntax('markdown')
@@ -48,7 +49,8 @@ Execute (Assert Syntax of escape typeface):
   AssertEqual 'VimwikiBold4'       , SyntaxAt(4, 14) . 4
   AssertEqual 'VimwikiItalic5'     , SyntaxAt(5, 14) . 5
   " See: #1303 where an escape start can close the region
-  AssertEqual '6'                  , SyntaxAt(6, 14) . 6
+  " Fixed removing vars.vim::expand_delimiter
+  AssertEqual 'VimwikiItalic6'     , SyntaxAt(6, 14) . 6
   AssertEqual 'VimwikiSuperScript7', SyntaxAt(7, 14) . 7
   AssertEqual 'VimwikiSubScript8'  , SyntaxAt(8, 14) . 8
   AssertEqual 'VimwikiDelText9'    , SyntaxAt(9, 14) . 9