Ensure prompt shows inside exclusive command evaluations

[lihaoyi]

Fixes #3677

• We need to introduce a Logger#withPromptUnpaused API, called internally within EvaluatorImpl, to make sure we turn on the prompt by default even when called within a exclusive=true command which turned it off

• We cannot use SystemStreams.original, because they do not go through Mills input redirection and so may arrive slightly out of order

• def run in build.mill has to be a Task.Command(exclusive = true) for testing stuff using mill dist.run to work properly

• I moved some of the gnarlier paused/pausedNoticed/stopped state from PromptLogger to an encapsulated PromperLogger#RunningState class where we can control the mutation and ensure it is done correctly

• Moved the periodic calling of readTerminalDims out of the synchronized block, only the subsequent storing of the data in memory needs to be synchronized