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:

styled-jsx/src/_utils.js

Line 530 in e41c6fe

let p = require(plugin)

What is the expected behavior?

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

Related: Thream/styled-jsx-plugin-sass#68