Move `filesystem ` source from verified-sources to dlt core

[burnash]

Background

The filesystem source, currently lives in the verified-sources repository. It's a critical component for many users of the dlt who load data from cloud and local storage. Given its importance and frequent usage, integrating this source directly into the dlt core simplifies the user experience and makes dependency management easier.

Proposal

Move the filesystem source from the verified-sources repository into the dlt core repository.

Tasks

• [ ] Migrate code
• [ ] Migrate tests
• [ ] Update documentation: installation, usage, and examples. Add migration docs
• [ ] Deprecate the filesystem source in the verified-sources repository

[burnash]

@rudolfix please feel free to update the description