Feature: Renaming Querier/Queries

[mpyw]

Renaming elements like structs in models is quite simple with gorename, so there's no pressing need to create a feature for this.

However, the case for Querier and Queries is different. These are incredibly generic names, and there may be times when you would like to use these names for elements not generated by sqlc. To accommodate such scenarios with flexibility, we will implement a feature that allows you to freely assign names to these.