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-initial-indent.vue

@@ -0,0 +1,28 @@
+<script>
+export default {}
+</script>
+
+<style lang="scss">
+  .a {
+    color: red;
+  }
+</style>
+
+<style>
+.b {
+  color: blue;
+}
+</style>
+
+<i18n>
+  {
+    "en": {
+      "hello": "Hello"
+    }
+  }
+</i18n>
+
+<i18n lang="yaml">
+en:
+  hello: Hello
+</i18n>