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-lang-attr.vim

@@ -0,0 +1,17 @@
+" lang in double or single quotes or without quotes, and
+" lang="typescript"
+let g:vim_vue_plugin_config = {
+      \'syntax': {
+      \   'template': ['html'],
+      \   'script': ['javascript', 'typescript'],
+      \   'style': ['css', 'scss', 'sass'],
+      \},
+      \}
+
+function! Check()
+  call ExpectEqual(GetVueSyntax(), 'html', 'syntax at line 1')
+  call ExpectBlock('let a', 'script', 'typescript', 'typescriptScriptBlock')
+  call ExpectBlock('let b', 'script', 'typescript', 'typescriptScriptBlock')
+  call ExpectBlock('$c', 'style', 'scss', 'cssScssStyleBlock')
+  call ExpectBlock('color: red', 'style', 'sass', 'sassStyleBlock')
+endfunction