Better error message for unsupported YAML file syntax

**Is your feature request related to a problem? Please describe.**
The `loadYamlContent()` expects single-document YAML content, and does not support document separators. If a multi-document YAML file is provided, the current error message surfaced is _Unable to parse literal YAML value. Please ensure that it is well-formed._

Example multi-document YAML (this is [technically](https://www.rfc-editor.org/rfc/rfc9512.html#section-3.2) valid YAML content):
```yml
abc: 'def'
---
ghi: 'jkl'
```

Bicep:
```bicep
var mydata = loadYamlContent('test.yaml') // Unable to parse literal YAML value. Please ensure that it is well-formed (bicep[BCP340]).
```

**Describe the solution you'd like**
If possible, it would be nice to surface a more specific error message, e.g. something along the lines of _Bicep does not support multi-document YAML content_.

Start by tracing the loadYamlContent() entry point and how it handles YAML document separators. Reproduce the example with two YAML documents, then verify that the resulting diagnostic identifies unsupported multi-document YAML content while single-document YAML still parses normally.

Written by the indexing model from the issue text.