Rename databaseName to databaseDriverName

[iamonah]

What

Rename parameter databaseName to databaseDriverName in NewWithInstance, NewWithDatabaseInstance functions and in the migrate object to clarify its purpose.

Why

The current name, databaseName, may confuse users into thinking it refers to the actual name of their database (e.g. "bank", "test_db"), when in reality it refers to the database driver name (e.g. "postgres", "mysql").

This change improves readability and aligns with the sourceName parameter, which correctly indicates that it's the name of the source driver.

@dhui would you mind reviewing when you get a chance? 🙏

[dhui]

@iamonah Thanks for the variable rename! We're lucky it's not an exported field so we're able to make this change.