Fix sqlc-dev#4195 by adding an emit_query_batch codegen option that creates a
QueryBatch type when using pgx v5.

When emit_query_batch: true is set in config (pgx/v5 only), sqlc generates:
- A QueryBatch struct wrapping pgx.Batch
- NewQueryBatch() constructor
- Queue* methods for each query (:one, :many, :exec, :execrows, :execresult)
- ExecuteBatch() on Queries to send all queued queries in one round-trip

This expands the batching support of existing :batchone/:batchmany/:batchexec annotations,
which required separate query definitions and only supported baching the
same query with different parameters. Batching multiple instances of the
same query with different parameters is also supported via the new
interface. While there is probably not a good reason to use both in the
same package, you can generate and use both without error for backwards
compatibility.

Key design decisions

- Follows pgx v5's recommended QueuedQuery callback pattern
- QueryBatch.Batch is exported so users can mix generated Queue* calls with custom pgx batch operations
- :exec queries have no callback (consistent with pgx - errors propagate via Close/ExecuteBatch)
- :one callbacks receive a bool indicating whether a row was found

Changes

- internal/codegen/golang/gen.go - Wire up EmitQueryBatch option, file generation, and validation
- internal/codegen/golang/imports.go - Add queryBatchImports() with import logic that skips struct field types (struct definitions live in query.sql.go, not the batch file). Extract queryUsesType() from inline
 closure for reuse.
- internal/codegen/golang/opts/options.go - Add EmitQueryBatch and OutputQueryBatchFileName options
- internal/codegen/golang/templates/pgx/queryBatchCode.tmpl - New template for all query batch methods
- internal/codegen/golang/templates/template.tmpl - Add queryBatchFile and queryBatchCode template definitions

Test coverage

- emit_query_batch - Full test with :one, :many, :exec, :execrows, :execresult
- emit_query_batch_db_arg - With emit_methods_with_db_argument: true
- emit_query_batch_minimal - Non-struct return types (verifies import handling for e.g. pgtype.Timestamptz)
- emit_query_batch_overrides - With custom type overrides (verifies batch file doesn't import types only used in struct fields)
- emit_query_batch_with_batch - Combines old-style :batchexec with emit_query_batch (verifies both batch.go and query_batch.sql.go coexist correctly)

Documentation

- docs/reference/config.md - Added emit_query_batch and output_query_batch_file_name
- docs/reference/query-annotations.md - New section with usage examples contrasting with :batch* annotations