ScalafmtRunner: use separate execution contexts

[kitbellew]

• in both core and dynamic runners, reading and writing is done the same way, with the only difference being formatting; hence, let's move I/O to shared runner
• to avoid reading all files first and only then formatting and writing them, let's create separate input and output execution contexts and use the former for reading and formatting, and the latter for writing
• finally, let's define parasitic execution context (one which executes immediately, without putting the task in a queue), for short tasks such as triggering .onComplete or updating task progress