Add methods to enable / disable PostGIS only if relevant

[gbuckingham89]

I noticed there are already methods for enabling (enablePostgis()) and disabling (disablePostgis()) PostGIS via a migration, but these throw an error if PostGIS is already enabled / disabled.

With the introduction of parallel testing in Laravel 8.25 the framework may create a new database for tests - therefore the enabling of PostGIS needs to be in a migration.

While the existing enablePostgis() method would work on the first run of the test suite, it causes an error on subsequent runs as the PostGIS extension is already enabled in the database.

Whilst this could be solved by using the --recreate-databases flag on each run of this test suite, I think it would be better to add methods to this package to handle this through the use of IF EXISTS and IF NOT EXISTS parameters. They maybe useful for other use cases too.

This PR adds two new methods, enablePostgisIfNotExists() and disablePostgisIfExists() and the relevant tests.

I've also updated the README - it already had instructions for manually enabling PostGIS, so I've added to it to cover using the new & old methods (which were undocumented) in a migration.

[mstaack]

Nice work, thank you.