tpl: Allow the partial template func to return any type

This commit adds support for return values in partials.

This means that you can now do this and similar:

    {{ $v := add . 42 }}
    {{ return $v }}

Partials without a `return` statement will be rendered as before.

This works for both `partial` and `partialCached`.

Fixes #5783

tpl/tplimpl/shortcodes.go

@@ -139,6 +139,18 @@ func templateNameAndVariants(name string) (string, []string) {
 	return name, variants
 }
 
+func resolveTemplateType(name string) templateType {
+	if isShortcode(name) {
+		return templateShortcode
+	}
+
+	if strings.Contains(name, "partials/") {
+		return templatePartial
+	}
+
+	return templateUndefined
+}
+
 func isShortcode(name string) bool {
 	return strings.Contains(name, "shortcodes/")
 }