fixes #112

BREAKING CHANGE: the library is now shipped as a hybrid CJS/ESM package. If you
are simply consuming the CLI tool or one of the existing integrations this will
not affect you.

For plugin developers and if you consume the API in any way the biggest change
is that the distributed source is now bundled and you can no longer access
individual files.

Typically something like:

```diff
-import foo from "html-validate/dist/foo";
+import { foo } from "html-validate"
```

Feel free to open an issue if some symbol you need isn't exported.

If your usage includes checking presence of rules use the `ruleExists` helper:

```diff
-try {
-  require("html-validate/dist/rules/attr-case");
-} catch (err) {
-  /* fallback */
-}
+import { ruleExists } from "html-validate";
+if (!ruleExists("attr-case")) {
+  /* fallback */
+}
```