Report error when assigning key to Starlark Builtin

**Describe the problem/challenge you have**
When assigning any [starlark builtin](https://github.com/google/starlark-go/blob/7a1108eaa0124ea025e567d9feb4e41aab4bb024/starlark/library.go#L77-L143) without invoking it to a yaml key, a panic is reported. While this is invalid code since its a reserved name in starlark, an error could be reported instead of panicing.

```starlark
key: #@ "".lower <- no parenthesis, just the function name
```

results in:

```go
- (p) unknown type *starlark.Builtin for conversion to go value (backtrace: goroutine 1 [running]:
in
defaults.yml:1 | key: #@ "".lower
...
```
**Describe the solution you'd like**
I would like to see an error that hints at a possible solution:

```go
- Unable to convert value: type *starlark.Builtin (did you mean .lower()?)
in
defaults.yml:1 | key: #@ "".lower
```

**Anything else you would like to add:**
The panic happens in [`starlark_value.go:108`](https://github.com/vmware-tanzu/carvel-ytt/blob/develop/pkg/template/core/starlark_value.go#L108). We should be able to catch this type to report an error.

---
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Start at pkg/template/core/starlark_value.go:108 and reproduce the issue with the provided defaults.yml snippet, where a Starlark builtin is assigned without parentheses. Trace the conversion path and add coverage for this input; done means it reports a conversion error with a possible .lower() hint instead of panicking.

Written by the indexing model from the issue text.