Everywhere: Add deprecated_ prefix to JsonValue::to_byte_string

`JsonValue::to_byte_string` has peculiar type-erasure semantics which is
not usually intended. Unfortunately, it also has a very stereotypical
name which does not warn about unexpected behavior. So let's prefix it
with `deprecated_` to make new code use `as_string` if it just wants to
get string value or `serialized<StringBuilder>` if it needs to do proper
serialization.

Userland/Shell/Shell.cpp

@@ -2067,7 +2067,7 @@ ErrorOr<Vector<Line::CompletionSuggestion>> Shell::complete_via_program_itself(s
                     dbgln("LibLine: Unhandled completion kind: {}", kind);
                 }
             } else {
-                suggestions.append(parsed.to_byte_string());
+                suggestions.append(parsed.deprecated_to_byte_string());
             }
 
             return IterationDecision::Continue;