[v7 only] Support ESM using native import

[mariusa]

Following up on #987
sequelize-cli v6 uses an import wrapper to support ESM.

For sequelize-cli v7, which drops node 10, please support ESM natively:

• use native import instead of the wrapper
• .sequelizerc no longer having module.exports and require
• have a .sequelizerc ESM example in docs

Thanks!

[ephys]

A .sequelizerc ESM example can already be added for v6 :) #990