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.

.gitignore

@@ -3,3 +3,4 @@ test.vue
 .DS_STORE
 test/deps/
 doc/tags
+test/other/