Add tests and GitHub Actions

Replace the Travis tests, which used a separate repo and no longer
ran, with tests in this repo. Each case in test/cases is indented as
a whole and should be unchanged, with optional syntax, fold and tag
checks. The cases are from the previous tests, plus script-setup.vue
for <script setup> keywords and a sync check in oneline.vim.

test/run.sh runs the cases with Vim in a pseudo terminal, as syntax in
Ex mode isn't the same, and Neovim headless.

test/cases/vue3.vue

@@ -0,0 +1,8 @@
+<style scoped>
+button {
+  color: v-bind(color);
+}
+.a :slotted(.b) :global(.c) :deep(.d) {
+  /* ... */
+}
+</style>