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/template.vim

@@ -0,0 +1,18 @@
+" Nested <template> and <docs> in <template>
+let g:vim_vue_plugin_config = {
+      \'syntax': {
+      \   'script': ['javascript'],
+      \   'template': ['html'],
+      \   'style': ['css'],
+      \   'docs': 'markdown',
+      \},
+      \}
+
+function! Check()
+  while search('/template', 'W')
+    call ExpectTag(line('.'), 'template')
+  endwhile
+  normal! gg
+  call ExpectNoSyntax('/docs', 'markdownDocsBlock')
+  call ExpectSyntax('/docs', 'markdownDocsBlock')
+endfunction