Support the new set-literal argument

[plnordquist]

Helm 3.12.0 seems to have introduced a set-literal argument to allow passing a literal value instead needing to escape strings with set or set-string. The commit that introduced this feature is at https://github.com/helm/helm/commit/0833318 and the PR is at https://github.com/helm/helm/pull/9182. The Helm Diff plugin should support this argument.