GeezerVison

This is a stacked PR on top of #4646, which adds :iter. This PR contains further tests, to keep that PR lean and reviewable.

Because Github doesn't do stacked PRs across a fork, it looks like this PR also contains #4646's changes. In order to review this commit I'd advise to look at the individual commits:

• 7f30b96
• fcefbdc

This adds test coverage for :iter at the same level as the other query types: one small case of its own, plus an :iter query in the existing cases for each Go option that changes the generated method.

PostgreSQL

• iter/postgresql/{stdlib,pgx/v5}: a struct result with a SQL comment, a scalar result with a parameter, and a DELETE … RETURNING, generated with emit_interface.
• An :iter query in wrap_errors, prepared_queries and comment_godoc_db_argument.
• invalid_queries_foo checks that :iter needs a RETURNING clause, like :one and :many.
• The "missing query type" error now lists :iter.

MySQL and SQLite

• An :iter query in wrap_errors (MySQL, SQLite), prepared_queries (MySQL) and sqlc_slice (MySQL).
• TestIterAuthors in the SQLite authors example. It runs the stdlib template against a real database, like the PostgreSQL authors test in Add the :iter query annotation #4646 does for pgx: the query only runs once ranging starts, ranging a second time reports an error, break releases the connection, and a cancelled query's error comes back from the returned func.

One template change. The sqlc.slice branch of the shared queryCodeStdExec helper started its output with a newline. Every other command calls the helper right after an opening {, so the newline never showed. :iter calls it after a statement, where it left a blank line. The branch now trims it, and no other golden changes.

The goldens of the existing cases only gain lines: the :iter method, its imports, and for prepared_queries the prepared statement.

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