Support caching of the formatter execution during startup

Per #3407 we should have a better way of running execution outside of the compiler. Maybe the compilation context should be used directly and/or has caching support directly in it as we run the same things over and over again here: https://github.com/compiler-explorer/compiler-explorer/blob/main/lib/handlers/formatting.js#L52 as each language makes a `new FormattingHandler`.