Do not omit string when context is `repl`

[ono-max]

We plan to support vscode debug visualizer. It sends Evaluate Request to debuggee and we need to return JSON based on vscode debug visualizer protocol. Although current debugger always tries to omit string, we don't want to omit it because JSON may be broken.

Thus, it would be good that we don't omit string when context is repl
I confirmed python debugger behaves the same way.