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-multiline-tag.vue

@@ -0,0 +1,20 @@
+<template>
+  <div
+    class="a"
+    id="b"
+  >
+    text
+    <input
+      type="text"
+      :value="name"
+    >
+    <img
+      src="a.png"
+      alt="a"
+    />
+    <input type="checkbox"
+      checked
+    >
+    <p>end</p>
+  </div>
+</template>