Add direction argument to table defaultGroup()

[tanthammar]

Description

When a table has ->defaultGroup('foo'), the initial grouping direction is hardcoded to asc.

// current
$this->tableGrouping = "{$this->getTable()->getDefaultGroup()->getId()}:asc";

This PR adds a $direction argument to defaultGroup(), mirroring the existing defaultSort() signature and naming convention.

// suggested change
$table->defaultGroup('created_at', 'desc');

Functional changes

• Did composer cs.
• Changes have been tested (3 new tests).
• Docs updated (packages/tables/docs/07-grouping.md).