Provide a way to either render output to stderr and/or include custom output to `--debug`

**Describe the problem/challenge you have**
Something blows up in some Starlark code in my template; but it's hard to get to the root cause because I can't see the values of expressions at runtime.

**Describe the solution you'd like**
The ability to include trace output during a `ytt` run. Even better if I could leave such tracing and be able to toggle it on and off from the command-line (e.g. include in `--debug` or introduce a flag `--trace`)

**Anything else you would like to add:**
The current workaround is to use `assert.fail()` in spots close to where the error occurs and include the desired context that way.

This idea has come up multiple times (in fact, hard to believe there isn't already an issue about this). Most recently in this Slack conversation: https://kubernetes.slack.com/archives/CH8KCCKA5/p1632938666479600
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To do:
- [ ] enumerate use-cases; describe a number of scenarios where runtime information/data/state would be useful.
- [ ] explore whether the generalized solution for addressing those use-cases would be best as an enhancement to `--debug` or a separate mechanism (e.g. another flag)
- [ ] explore how complex/custom Starlark values (structs, yamlfragments, ...) would be presented.
- [ ] explore what security concerns might be present.

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We are also happy to receive and review Pull Requests if you want to help working on this issue.

Start with the issue's use-case checklist and inspect how ytt's --debug option and command-line flags expose runtime information during a Starlark evaluation. Document scenarios, output for complex Starlark values, and security concerns before choosing between extending --debug and adding a separate tracing mechanism. Done means the scope and proposed interface are agreed, rather than merely implementing one output format.

Written by the indexing model from the issue text.