Update chat's newEditSession command to take an input prompt

[cmbrose]

For standard chat, there is a workbench.action.chat.sendToNewChat command which takes an input value and automatically submits the value.

However, that command doesn't exist for edit sessions. There is, however, already the workbench.action.chat.newEditSession command which will create a new, empty edit session. This PR extends that command to take 2 new optional arguments:

• agentMode - boolean to start the new session in agent mode or not
• inputValue - the prompt text to write and then submit

There were a lot of similar commands that I looked at for this PR, as well as just making a new command to only accept input text. I ended up going this route because the newEditSession logic already had support for warning when overwriting an in-progress session and similar logic that would otherwise need to be duplicated.