Include _get_source_code into Function class

### Describe the desired feature

even if not used by any printer because of how cli formatting works with PrettyTables, it's actually a useful method to have accessible when working with Python API directly, currently this method is located only in

https://github.com/crytic/slither/blob/e3dcf1ecd3e9de60da046de471c5663ab637993a/slither/printers/summary/constructor_calls.py#L11

forcing any python script to import it using the following path

`from slither.printers.summary.constructor_calls import _get_source_code
`

`_get_source_code` function would be better located inside of a [Function class](https://github.com/crytic/slither/blob/master/slither/core/declarations/function.py). increased function visibility in the codebase will also save developers time on asking chatgpt how to regex solidity function.