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()

test/cases/syntax-merge.vim

@@ -0,0 +1,10 @@
+" Each key of syntax replaces the default of the same key, so template and
+" style keep html and css
+let g:vim_vue_plugin_config = {'syntax': {'script': ['javascript', 'typescript']}}
+let g:test_flatten = 1
+
+function! Check()
+  call ExpectBlock('<div>', 'template', 'html', 'htmlTemplateBlock')
+  call ExpectBlock('let count', 'script', 'typescript', 'typescriptScriptBlock')
+  call ExpectBlock('color: red', 'style', 'css', 'cssStyleBlock')
+endfunction

test/cases/syntax-merge.vue

@@ -0,0 +1,13 @@
+<template>
+  <div>{{ count }}</div>
+</template>
+
+<script lang="ts">
+let count: number = 0
+</script>
+
+<style>
+p {
+  color: red;
+}
+</style>