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/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>