Add default lint level for default features

[Jake-Shadle]

This PR changes the configuration of features, as, in #434, the configuration followed exactly what was proposed in #226, but on reflection that configuration would be super confusing, so now bans.features is its own top-level option.

This PR also adds two new configuration options, bans.workspace-default-features and bans.external-default-features which allows configuration of a default lint option for either workspace or external (non-workspace) crates. This default can then be overriden specifically for a crate, eg.

[bans]
workspace-default-features = 'deny'

[[bans.features]]
name = "a-workspace-crate"
allow = ["default"]

Would allow default features on a-workspace-crate, but deny them on any other workspace crate.

Also adds more testing of feature banning, to where I'm fairly confident this resolves #226.

Resolves: #206
Resolves: #226
Closes: #347