tpl: Narrow down the usage of plain text shortcodes when rendering HTML

After this commit, if you want to resolve `layouts/_shortcodes/myshortcode.txt` when rendering HTML content, you need to use the `{{%` shortcode delimiter:

```
{{% myshortcode %}}
```

This should be what people would do anyway, but we have also as part of this improved the error message to inform about what needs to be done.

Note that this is not relevant for partials.

Fixes #13698

tpl/tplimpl/templatedescriptor.go

@@ -37,6 +37,7 @@ type TemplateDescriptor struct {
 	// Misc.
 	LayoutFromUserMustMatch bool // If set, we only look for the exact layout.
 	IsPlainText             bool // Whether this is a plain text template.
+	AlwaysAllowPlainText    bool // Whether to e.g. allow plain text templates to be rendered in HTML.
 }
 
 func (d *TemplateDescriptor) normalizeFromFile() {
@@ -64,7 +65,7 @@ func (s descriptorHandler) compareDescriptors(category Category, isEmbedded bool
 		return weightNoMatch
 	}
 
-	w := this.doCompare(category, isEmbedded, s.opts.DefaultContentLanguage, other)
+	w := this.doCompare(category, s.opts.DefaultContentLanguage, other)
 
 	if w.w1 <= 0 {
 		if category == CategoryMarkup && (this.Variant1 == other.Variant1) && (this.Variant2 == other.Variant2 || this.Variant2 != "" && other.Variant2 == "") {
@@ -74,7 +75,12 @@ func (s descriptorHandler) compareDescriptors(category Category, isEmbedded bool
 			}
 
 			w.w1 = 1
-			return w
+		}
+
+		if category == CategoryShortcode {
+			if (this.IsPlainText == other.IsPlainText || !other.IsPlainText) || this.AlwaysAllowPlainText {
+				w.w1 = 1
+			}
 		}
 	}
 
@@ -82,13 +88,16 @@ func (s descriptorHandler) compareDescriptors(category Category, isEmbedded bool
 }
 
 //lint:ignore ST1006 this vs other makes it easier to reason about.
-func (this TemplateDescriptor) doCompare(category Category, isEmbedded bool, defaultContentLanguage string, other TemplateDescriptor) weight {
+func (this TemplateDescriptor) doCompare(category Category, defaultContentLanguage string, other TemplateDescriptor) weight {
 	w := weightNoMatch
 
-	// HTML in plain text is OK, but not the other way around.
-	if other.IsPlainText && !this.IsPlainText {
-		return w
+	if !this.AlwaysAllowPlainText {
+		// HTML in plain text is OK, but not the other way around.
+		if other.IsPlainText && !this.IsPlainText {
+			return w
+		}
 	}
+
 	if other.Kind != "" && other.Kind != this.Kind {
 		return w
 	}