Add a task instancePolicy to task runOptions.

When a task has reached the max number of instances it can have (default is 1, can be changed with `instanceLimit`) the `instancePolicy` determines what happens when another instance is started.

History: https://github.com/microsoft/vscode/issues/32264
PR for `insanceLimit` which is relevant to anyone interested in implementing `instancePolicy`: https://github.com/microsoft/vscode/pull/89872