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/oneline.vue

@@ -0,0 +1,12 @@
+<script src="./tmp"> </script>
+
+<style lang="css">
+p {
+  color: red;
+}
+</style>
+
+
+<style>p { color: red; }</style>
+
+<template><div>Hello</div></template>