Minimal configuration errors due to missing tool.vulcan configuration

[danjones1618]

With the minimal pyproject.toml listed in the README.md, vulcan errors with the following:

$ vulcan add textual

Traceback (most recent call last):
  File "/home/dan/.local/bin/vulcan", line 8, in <module>
    sys.exit(main())
  File "/home/dan/.local/pipx/venvs/vulcan-py/lib/python3.10/site-packages/click/core.py", line 1130, in __call__
    return self.main(*args, **kwargs)
  File "/home/dan/.local/pipx/venvs/vulcan-py/lib/python3.10/site-packages/click/core.py", line 1055, in main
    rv = self.invoke(ctx)
  File "/home/dan/.local/pipx/venvs/vulcan-py/lib/python3.10/site-packages/click/core.py", line 1654, in invoke
    super().invoke(ctx)
  File "/home/dan/.local/pipx/venvs/vulcan-py/lib/python3.10/site-packages/click/core.py", line 1404, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/home/dan/.local/pipx/venvs/vulcan-py/lib/python3.10/site-packages/click/core.py", line 760, in invoke
    return __callback(*args, **kwargs)
  File "/home/dan/.local/pipx/venvs/vulcan-py/lib/python3.10/site-packages/click/decorators.py", line 26, in new_func
    return f(get_current_context(), *args, **kwargs)
  File "/home/dan/.local/pipx/venvs/vulcan-py/lib/python3.10/site-packages/vulcan/cli.py", line 54, in main
    ctx.obj = Vulcan.from_source(Path().absolute(), fail_on_missing_lock=False)
  File "/home/dan/.local/pipx/venvs/vulcan-py/lib/python3.10/site-packages/vulcan/__init__.py", line 75, in from_source
    config = all_config['tool']['vulcan']  # type: ignore[index]
  File "/home/dan/.local/pipx/venvs/vulcan-py/lib/python3.10/site-packages/tomlkit/container.py", line 650, in __getitem__
    raise NonExistentKey(key)
tomlkit.exceptions.NonExistentKey: 'Key "tool" does not exist.'

I think the appropriate solution is to not require an empty [tool.vulcan] section as apposed to adding the blank section to the configuration.

On a side note, "dependencies" should probably be added to the dynamic array