Test all features

Add cases for attribute highlight on and off, block syntax by lang
and emmet-vim group names, OnChangeVueSyntax(), matchit, .wpy,
initial_indent as syntax and tag.syntax, and syntax after reloading.

Also check indenting a file after removing all indent with
g:test_flatten, which fails before the end tag fix as closing tags at
column 0 were taken as block end tags.

test/cases/context.vue

@@ -0,0 +1,11 @@
+<template>
+  <div></div>
+</template>
+
+<script lang="ts">
+const a: number = 1
+</script>
+
+<style lang="scss">
+$a: 1px;
+</style>