feat(codegen): Insert comments in interfaces

[sbres]

Related to #1400 I've added the comments into the interface generation the same way it's done for the auto-generated methods.

I've compiled sqlc using make sqlc-dev. And ran it on this repo https://github.com/sbres/sqlc_example.

The generated output went from:

type Querier interface {
	CheckUserExist(ctx context.Context, email string) (bool, error)
	CreateNewUser(ctx context.Context, arg CreateNewUserParams) (int32, error)
}

To

type Querier interface {
	// CheckUserExist will check if the user exist, retuns true if the exists
	CheckUserExist(ctx context.Context, email string) (bool, error)
	// CreateNewUser will create a new user
	CreateNewUser(ctx context.Context, arg CreateNewUserParams) (int32, error)
}

I was not able to run the full suite of tests.