"#@data.values" [no space after @] should error with a helpful hint

**Describe the problem/challenge you have**
users may accidentally forget to have a space in `foo: #@data.values` after `@` and be confused why the data value is not showing up in the output.

(https://twitter.com/stylishandy/status/1455051198935957514?s=20)

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Start by reproducing the issue in the ytt CLI with `foo: #@data.values` and compare it with the spaced form. Trace how the template parser handles this expression; done means the no-space form produces a clear error with a helpful hint instead of silently failing to show the data value.

Written by the indexing model from the issue text.