Improve error if `existing` goes in the wrong place

**Is your feature request related to a problem? Please describe.**
I use `existing` enough to know the feature is there, but not enough to remember where exactly the `existing` keyword goes - I often find myself putting it after the equals sign.

**Describe the solution you'd like**
Could you detect this scenario and provide a more specific error message. (and maybe even a fix to move `existing` into the right place?

Reproduce the reported error in a minimal Bicep resource declaration, placing `existing` after the equals sign as described. Trace the parser or compiler diagnostic for the misplaced keyword; done means the scenario produces a specific, actionable error message, with relocation behavior included only if the project accepts it.

Written by the indexing model from the issue text.