Add support for implementation variants

An implementation container should be able to signal it's running itself in multiple configurations, and pass back values for each.

E.g. ajv has a strict mode which can be enabled or disabled, and we should collect results from both.

The report should then be updated to group variants together. We should display the "default" variant first, where default is defined to be the variant one gets when passing minimal or no additional configuration, or if configuration is always necessary, the one one gets when following the first documented or most recommended configuration from the implementation's documentation.

Worth considering whether to treat versions (of the implementation) as variants, or whether these are separate axes.

(The presumption is that we can handle this within the same runtime rather than running multiple instances of a container, which we already support)

Though it may complicate the just-mentioned presumption, we may also wish to support testing CLIs as if they were variants of the implementations they're based on.

No files or tests are named; first map the existing implementation-container, report-generation, and multiple-instance flows, including current CLI handling. Done means configurations can return separate values, variants are grouped with the default first, and the version and same-runtime questions have an agreed implementation scope.

Written by the indexing model from the issue text.