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-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>