Add shared test cases with vim-svelte-plugin

Name cases for scenarios also in vim-svelte-plugin as shared-*, adding
one for multiline tags. test/shared.sh lists shared cases in only one
of the plugins, as a warning in a separate job on GitHub Actions.

test/cases/shared-lang-attr.vue

@@ -0,0 +1,20 @@
+<template lang=html>
+  <div>x</div>
+</template>
+
+<script lang=ts>
+let a: number = 1
+</script>
+
+<script lang="typescript">
+let b: number = 2
+</script>
+
+<style lang='scss'>
+$c: 1px;
+</style>
+
+<style lang=sass>
+p
+  color: red
+</style>