feat: add local subcommand to compare two chart folders

[bo0tzz]

This adds a new local subcommand that diffs two local chart directories. My intended usecase is mainly for development and CI/CD, to see what effects a change has.

This code was written entirely by an LLM; if that is a dealbreaker, feel free to just close this PR outright.

If there is appetite for it, I'd like to next build on top of this with a git subcommand that diffs two git references against eachother (by checking them out to a temporary folder and then running local against them).