Added Always First Functionality

This pull request resolves #164.

This enables the new file type AF__*.sql which is processed before all other file types.
This option defaults to False but can be enabled with the configuration -af or --always-first. This ensures that these files can be selectively executed depending on the environment.
- e.g. In the CI/CD pipeline, you enable this configuration option when you need to clone the production environment to create the QA environment. However, this option is turned off for the elevation from QA to Prod because no cloning operation is taking place.
Tests were added to ensure the functionality works as designed. These tests cove:
- Returning the proper number of files executed when the option is turned on or off.
- Returning expected errors based on naming conventions.
- Validating the appropriate data for the Change History Table.
- Validating the appropriate execution order (AF->V->R->A) when the option is turned on.
- Validating the appropriate execution order (V->R->A) when the option is turned off.
Added this new functionality to the README documentation.

Snowflake-Labs / schemachange