Error when a non-template annotation is used for setting the value of a node

**Describe the problem/challenge you have**
When I accidentally write non-template-code for an inline comment in my template, the resulting value is `null`.

Example:

```
#@ load("@ytt:data", "data")
---
akoOperator:
config:
avi_control_plane_ha_provider: #@ data.values.AVI_CONTROL_PLANE_HA_PROVIDER
#@ if data.values.AVI_NSXT_T1LR:
avi_nsxt_t1_lr: #@data.values.AVI_NSXT_T1LR
#@ end
```
yields:
```
akoOperator:
config:
avi_control_plane_ha_provider: true
avi_nsxt_t1_lr: null
```
_(from: https://carvel.dev/ytt/#gist:https://gist.github.com/XudongLiuHarold/d99b14e347601b9254e86330e99aa9ad)_

This is a bad experience because `ytt` returns a value giving me the impression that the template was well-formed and valid.

**Describe the solution you'd like**
Report an error when any other annotation _other_ than `@template/code` or `@template/value` (or their short-hand `@ `) is provided for an inline comment.

Bonus points if we can _reliably_ detect cases like the one above and be able to suggest ("Did you mean `avi_nsxt_t1_lr: #@ data.values.AVI_NSXT_T1LR`?")

**Anything else you would like to add:**

Consider: should we have a list of "reserved" annotations that are not allowed. e.g. @for, @if, @data.values, @end ... and detect these generally?

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

Start by reproducing the inline-comment example from the issue in ytt and inspect the annotation handling path that turns the non-template annotation into null. Determine how inline annotations are distinguished from @template/code, @template/value, and shorthand @ forms. Done means invalid inline annotations produce an error, with tests covering the example and any reserved-annotation behavior agreed by the project.

Written by the indexing model from the issue text.