Changelogger Enhancement: Add confirmation messages for successful commands

### Impacted plugin

None / Other

### What

Add confirmation message when a command is successful

### How

When working on implementing Jetpack Changelogger for WCPay https://github.com/Automattic/woocommerce-payments/pull/3685#discussion_r804523415, we have a feedback that it would be less confusing if we add confirmation messages for successful commands.

Currently, running `./vendor/bin/changelogger write` or `./vendor/bin/changelogger add` simply finishes itself without any confirmation.

For example, I checked the `add` command code [src1](https://github.com/Automattic/jetpack/blob/master/projects/packages/changelogger/src/AddCommand.php#L76-L90) and [src2](https://github.com/Automattic/jetpack/blob/c2f947b546923a4e428e1d886936d7decdf46f32/projects/packages/changelogger/src/AddCommand.php#L335) there is no option to show the successful messages.

Start by reading projects/packages/changelogger/src/AddCommand.php around the referenced command implementation, then run ./vendor/bin/changelogger add and ./vendor/bin/changelogger write to observe their current output. Done means both successful commands clearly report confirmation messages without changing their existing behavior.

Written by the indexing model from the issue text.