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/script-setup.vue

@@ -0,0 +1,15 @@
+<template>
+  <div ref="el">{{ props.foo }}</div>
+</template>
+
+<script setup lang="ts">
+const props = withDefaults(defineProps<{ foo?: string }>(), {
+  foo: 'a',
+})
+const emit = defineEmits<{ (e: 'change', id: number): void }>()
+const model = defineModel<string>()
+const el = useTemplateRef('el')
+const count = ref<number>(1)
+const small = count.value < 3
+defineExpose({ el })
+</script>