not working with snowpack 3

Hello! This lib seems to be not working with snowpack 3.3.7. I am getting this error:

```
Error [ERR_REQUIRE_ESM]: require() of ES Module
node_modules/@mdx-js/mdx/index.js
from
node_modules/snowpack-plugin-mdx/dist/main/index.js
not supported.
Instead change the require of
node_modules/@mdx-js/mdx/index.js
in
node_modules/snowpack-plugin-mdx/dist/main/index.js
to a dynamic import() which is available in all CommonJS modules.
```

I guess snowpack does not like the commonJS style import...
Is there any workaround?

Reproduce the failure with Snowpack 3.3.7 and inspect the plugin entry point involved in the reported stack trace, node_modules/snowpack-plugin-mdx/dist/main/index.js. Compare how it loads @mdx-js/mdx and verify the plugin can process MDX without ERR_REQUIRE_ESM; the issue does not name a repository source file or test to run.

Written by the indexing model from the issue text.