Add terminalDisposed Signal to ILiteTerminalAPIClient

[ianthomas23]

Add terminalDisposed Signal to ILiteTerminalAPIClient, which is emitted when a terminal/shell is disposed and it passes the terminal name which is the same as the cockle.Shell shellId. Can be used something like this:

liteTerminalAPIClient.terminalDisposed.connect((_, name) => console.log(`${name} disposed!`));

The use case for this is a JupyterLite extension that contains a cockle external command. The same command would be used for all opened terminals but it is possible that the command would want to cache some information on a per-terminal basis, and this signal can be used to identify when cached information for a particular terminal can be deleted.

It is possible that it might also be useful to have another new signal for a terminal being created, but I don't yet have a real use case for this so it is not included here.

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