Make the new driver the default

[JanJakes]

Currently, the new AST-based SQLite driver is behind a feature flag (constant WP_SQLITE_AST_DRIVER).

Once we're confident, we can switch to it by default, we can make the default behavior for when the WP_SQLITE_AST_DRIVER constant is not defined to be the same as define( 'WP_SQLITE_AST_DRIVER', true );.

When this is done, search for WP_SQLITE_AST_DRIVER in the Playground repo and remove any explicit definitions of the constant.

Later on, we'll remove the old driver entirely, but that is not the aim of this ticket.