Kernel: Add KLexicalPath::try_join and use it

This adds KLexicalPath::try_join(). As this cannot be done without
allocation, it uses KString and can fail. This patch also uses it at one
place. All the other cases of String::formatted("{}/{}", ...) currently
rely on the return value being a String, which means they cannot easily
be converted to use the new API.

Kernel/KLexicalPath.h

@@ -7,6 +7,7 @@
 #pragma once
 
 #include <AK/StringView.h>
+#include <Kernel/KString.h>
 
 namespace Kernel::KLexicalPath {
 
@@ -16,4 +17,6 @@ StringView basename(StringView const&);
 StringView dirname(StringView const&);
 Vector<StringView> parts(StringView const&);
 
+OwnPtr<KString> try_join(StringView const&, StringView const&);
+
 }