This is mainly to reach parity with how stable handles such cases for
skins specifically. It was brought to our attention after finding a skin
with a malformed `\[General]` tag which did not break further parsing of
the file on stable, but did on lazer.
Note that stable defaults to an "unknown" section for beatmaps, but
functionally I can't think of a regression case from changing this to a
"general" default.
`IHasComboColours` was already mutable (via a strange
`AddComboColours()` method) and exposing a straight list is easier to
work with. `IHasCustomColours` is also similarly externally mutable (in
a way which is not easily removable).