**Feature summary:**
Add a `returnType` field to TemplateData to describe the type of value returned by a template. The field should accept the same values currently supported by parameter `type` fields (e.g. `string`, `content`, `boolean`, `number`, `date`, etc.).
For example:
```
<templatedata>{
"description": "Returns the formatted name of a person.",
"returnType": "string",
"params": {
"name": {
"label": "Name",
"type": "string"
}
}
}</templatedata>
```
**Use case(s):**
In some contexts, only particular types of values can be inserted. For example, inside an HTML attribute, a template returning a value of type `content` may produce invalid output:
```html
<span class="<b>Hello world</b>">...</span>
```
where `<b>Hello world</b>` is the expansion of a template. In such a context, a template returning a plain string would be appropriate, while one returning arbitrary content would not.
Currently, TemplateData can describe the expected type of each parameter, but it cannot describe the type of a template's output. This means that tools consuming TemplateData have no way to know whether a template returns a string, content, a boolean value, or another kind of value.
Adding a `returnType` field would allow template authors to provide this information in a machine-readable way.
**Benefits:**
A `returnType` field would make TemplateData a more complete description of a template's interface. It would allow editing tools, documentation generators, and other consumers of TemplateData to better understand how templates can be used and combined.
This would also improve consistency by reusing the existing TemplateData type vocabulary rather than introducing a separate system for template output values.