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 [].

README.md

@@ -92,6 +92,7 @@ It has the following options
         - `lang="..."` on block tag decides the effective syntax
         - When no valid `lang="..."` is present, the first syntax in the list will be used.
         - By default, only syntax files from `['$VIMRUNTIME', '$VIM/vimfiles', '$HOME/.vim']` are loaded. If none is found, then **full** syntax files, including those from plugins, will be loaded
+    - Each key replaces the default of the same key, so `{'syntax': {'script': ['javascript', 'typescript']}}` keeps `template` as `['html']` and `style` as `['css']`. Set a key to `[]` to remove the block
 - **full_syntax**: *string list*. Syntax whose **full** syntax files will always be loaded
 - **initial_indent**: *string list*. Tag/syntax with initial one tab indent. The format can be `tag.syntax`, `tag`, or `syntax`