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.vim

@@ -0,0 +1,10 @@
+" initial_indent as `syntax` and `tag.syntax`
+let g:vim_vue_plugin_config = {
+      \'syntax': {
+      \   'script': ['javascript'],
+      \   'style': ['css', 'scss'],
+      \   'i18n': ['json', 'yaml'],
+      \},
+      \'initial_indent': ['scss', 'i18n.json'],
+      \}
+let g:test_flatten = 1