Merge syntax config per block

syntax in g:vim_vue_plugin_config replaced the whole default, so
{'syntax': {'script': ['javascript', 'typescript']}} removed template
and style, which then had no highlight or indent. Replace the default
of each key separately, like vim-svelte-plugin. A block can be removed
by setting it to [].

test/cases/no-css.vim

@@ -1,5 +1,8 @@
-" Without css syntax, like only script in config
-let g:vim_vue_plugin_config = {'syntax': {'script': ['javascript']}}
+" Without css syntax, like only script in config, removing template and style
+" by []
+let g:vim_vue_plugin_config = {
+      \'syntax': {'script': ['javascript'], 'template': [], 'style': []},
+      \}
 let g:test_flatten = 1
 
 function! Check()