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-shiftwidth-zero.vue

@@ -0,0 +1,24 @@
+<template>
+    <div
+        class="a"
+    >
+        <input
+            type="text"
+        >
+        <p :class="{ a: b }">x</p>
+    </div>
+</template>
+
+<script>
+export default {
+    data() {
+        return { a: 1 }
+    },
+}
+</script>
+
+<style>
+p {
+    color: red;
+}
+</style>