Support ESM import syntax for plugins

[theoludwig]

What is the current behavior?

Currently, styled-jsx plugins can't be written with ESM imports syntax because it requires the import instead of using dynamic import syntax.

The problematic line of code is: https://github.com/vercel/styled-jsx/blob/e41c6fe488b7c25ddc2a8033106f1df424986ccc/src/_utils.js#L530

What is the expected behavior?

styled-jsx should be able to work with plugins written with ESM.

Related: https://github.com/Thream/styled-jsx-plugin-sass/issues/68