Simplify ParsingRun error reporting functions

[lihaoyi]

• Rename def aggregateMsg with reportAggregateMsg and def aggregateTerminal with def reportTerminalMsg, and failureGroupAggregate and failureTerminalAggregate with aggregateParserMsgs and terminalParserMsgs, for consistency with def longAggregateMsg and def longTerminalMsg

• Tighted up error reporting logic to avoid parse failures that happened earlier when index < traceIndex from being shown in the final failureAggregates output, which is intended to only show parse failures that occurred when index == traceIndex (with associated tests)

• Consolidated all def report*Msg methods into a single def reportParseMsg0 to avoid unnecessary code duplication and divergence

• Add some implicit constructors for Msgs to clean up callsites

• Moved long explanation of how error reporting works to the readme docsite, and fixed sbt readme/run to work with new version of Mill

• Inline & and ! operators on Scala 3 for perf