Add a way to enable/disable echo in TermImpl echoHandler

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Describe the feature

Allow enable/disable echoing input characters back to the remote client.

Use cases

This allows temporarily disabling echo, for example to input a password, without replacing echoHandler and losing built-in readline functionality.

Contribution

Proposed implementation in TermImpl.java:

• Add a new class field:

  private boolean echoOn = true;

• Add an if block in the constructor:

    echoHandler = codePoints -> {
      // Echo
      if (echoOn) {
        echo(codePoints);
      }
      readline.queueEvent(codePoints);
    };

• Add a setter:

  public void setEchoOn(boolean echoOn) {
    this.echoOn = echoOn;
  }

Start in TermImpl.java and inspect the echoHandler in its constructor. Add a way to toggle echoing without replacing the handler, while preserving readline event queuing. Done means input echo can be enabled or disabled for cases such as password entry.