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/fold.vim

@@ -0,0 +1,15 @@
+let g:vim_vue_plugin_config = {
+      \'syntax': {
+      \   'script': ['javascript'],
+      \   'template': ['html'],
+      \   'style': ['css'],
+      \},
+      \'foldexpr': 1,
+      \}
+
+function! Check()
+  let levels = [1, 2, 1, 1, 1, 2, 3, 4, 4, 3, 3, 1, 1, 1, 1, 1, 2, 2, 1, 1]
+  for lnum in range(1, len(levels))
+    call ExpectFoldlevel(lnum, levels[lnum - 1])
+  endfor
+endfunction