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 helm/helm@0833318 and the PR is at helm/helm#9182. The Helm Diff plugin should support this argument.

[interone-ms]

Relevant for my usecase as well, I'd be grateful if this could be implemented!

[interone-ms]

@plnordquist I had an attempt at this myself, if you're still interested you can fetch my fork's master branch and try it out. For me, it worked.