Add public constant with value "pgx"

Enter a constant for the driver name so that it can be used when connecting to the database via "database/sql" in the standard connection command.
```GO
import "github.com/jackc/pgx/v5/stdlib"
...
// stdlib.DriverName = "pgx"
db, err := sql.Open(stdlib.DriverName, os.Getenv("DATABASE_URL"))
...
```
The string "pgx" is used several times in the library, the introduction of a constant seems appropriate.

Start in the github.com/jackc/pgx/v5/stdlib package, where the database/sql driver is exposed. Add the public driver-name constant with value "pgx", then verify that sql.Open accepts it in the standard connection example or relevant tests.

Written by the indexing model from the issue text.