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/example.vim

@@ -0,0 +1,15 @@
+" Uses syntax plugins in test/deps
+let g:vim_vue_plugin_config = {
+      \'syntax': {
+      \   'template': ['html', 'pug'],
+      \   'script': ['javascript', 'typescript', 'coffee'],
+      \   'style': ['css', 'scss', 'sass', 'less', 'stylus'],
+      \   'docs': 'markdown',
+      \   'i18n': ['json', 'yaml'],
+      \   'route': 'json',
+      \},
+      \'attribute': 1,
+      \'keyword': 1,
+      \'foldexpr': 1,
+      \'full_syntax': ['scss', 'less'],
+      \}