Add configuration option to specify slither target

Hi everyone!

Maybe I don't know where to look, but it seems to me that there is currently no option to specify the target directory for slither. I can run slither from my CLI with `slither src` where `src` is the `target`-folder containing the Solidity files. But the VSCode extension is always trying to run `slither .` which will ultimately throw the following error:

~~~
⸻ Starting analysis ⸻
Error: Error in workspace "/home/raoul/project":
Traceback (most recent call last):
File "/home/raoul/.local/lib/python3.8/site-packages/slither/__main__.py", line 743, in main_impl
) = process_all(filename, args, detector_classes, printer_classes)
File "/home/raoul/.local/lib/python3.8/site-packages/slither/__main__.py", line 73, in process_all
compilations = compile_all(target, **vars(args))
File "/home/raoul/.local/lib/python3.8/site-packages/crytic_compile/crytic_compile.py", line 637, in compile_all
compilations.append(CryticCompile(target, **kwargs))
File "/home/raoul/.local/lib/python3.8/site-packages/crytic_compile/crytic_compile.py", line 117, in __init__
self._compile(**kwargs)
File "/home/raoul/.local/lib/python3.8/site-packages/crytic_compile/crytic_compile.py", line 548, in _compile
self._platform.compile(self, **kwargs)
File "/home/raoul/.local/lib/python3.8/site-packages/crytic_compile/platform/hardhat.py", line 87, in compile
os.listdir(build_directory), key=lambda x: os.path.getmtime(Path(build_directory, x))
FileNotFoundError: [Errno 2] No such file or directory: 'artifacts/build-info'

⸻ Analysis: 0 succeeded, 1 failed, 0 skipped ⸻
Refreshing explorer...
Loaded 0 issues, displaying 0
~~~

The problem seems to be the following line, which passes `.` as the hardcoded target to slither:

https://github.com/crytic/slither-vscode/blob/9cbf906ebac9ef9b61964609beda17a646b0bac7/src/slither.ts#L101

It would be nice to have a workspace-folder configuration option to specify the target manually.