Fix indentation in highlight shortcode

This commit adds a new `.InnerDeindent` method to the shortcode context, which is `.Inner` with any
indendation removed. This is then used in the built-in `highlight` shortcode to prevent the extra
whitespace getting hightlighted.

Fixes #4717

tpl/tplimpl/template_ast_transformers.go

@@ -272,7 +272,7 @@ func (c *templateContext) collectInner(n *parse.CommandNode) {
 			idents = nt.Ident
 		}
 
-		if c.hasIdent(idents, "Inner") {
+		if c.hasIdent(idents, "Inner") || c.hasIdent(idents, "InnerDeindent") {
 			c.t.parseInfo.IsInner = true
 			break
 		}