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-no-syntax-file.vue

@@ -0,0 +1,15 @@
+<template>
+  <div>x</div>
+</template>
+
+<style lang="postcss">
+p
+  color: red
+  a
+    color: blue
+</style>
+
+<i18n lang="toml">
+[en]
+  hello = "hi"
+</i18n>