Add syntax highlighting tests for VimL

Adds a syntax highlighting test for VimL
with source file based on parts of my own configuration
changed to cover as much of syntax as possible.

**NOTES:**
Last line of source (`syntax enable`) does not get highlighted,
since `syntax` keyword is not part of highlighting rules.

Related to #1213

tests/syntax-tests/source/VimL/source.vim

@@ -0,0 +1,79 @@
+if &compatible
+  set nocompatible
+endif
+
+if has('win32') || has ('win64')
+  let $VIMHOME = $HOME . "/vimfiles"
+elseif v:false && v:true
+  echo "Can't get here"
+else
+  let $VIMHOME = $HOME . "/.vim"
+endif
+
+" show existing tab with 2 spaces width
+set tabstop=2
+" when indenting with '>', use 2 spaces width
+set shiftwidth=2
+" always set autoindenting on
+set autoindent
+
+autocmd VimEnter * echo "Hello Vim"
+
+" Allow :W and :Wq to save too
+command! Wq :wq
+command! W :w
+
+augroup vimrc
+    autocmd!
+    autocmd FileType * echo "New filetype"
+augroup END
+
+function! s:echo(what)
+  return a:what
+endfunction
+
+function! HelloWorld(name)
+  let l:function_local = "function_local_var"
+  let l:parts = split(l:function_local, "_")
+  let l:greeting = "Hello " . a:name
+  return s:echo(l:greeting)
+endfunction
+
+function! source#Hello()
+  return "Hello from namespace"
+endfunction
+
+function! EchoFunc(...)
+  for s in a:000
+    echon ' ' . s
+  endfor
+endfunction
+
+imap <C-h> <C-r>=HelloWorld("World")<CR>
+
+command! -nargs=? Echo :call EchoFunc(<args>)
+
+let g:global = "global var"
+let s:script_var = "script var"
+let w:window_var = "window war"
+let b:buffer_var = "buffer war"
+let t:tab_var = "tab war"
+echo v:false
+
+3 + 5
+
+echo "Hello" ==# "Hello2"
+echo "Hello" ==? "Hello2"
+echo "Hello" == "Hello2"
+echo "Hello" is "Hello2"
+echo "Hello" isnot "Hello2"
+echo "Hello" =~ "Hello2"
+echo "Hello" !~ "Hello2"
+
+let g:dict = {}
+let g:dict.item = ['l1', 'l2']
+
+let g:dict2 = {'dict_item': ['l1', 'l2'], 'di2': 'x'}
+
+filetype plugin indent on
+syntax enable