Try using -Ymagic-offset-header to fixup line numbers

[lihaoyi]

This flag lets us remove a lot of our sketchy user-land workarounds to fix the filename and line numbers of errors:

• AsmPositionUpdater to which handled runtime exception error positions
• PositionalMapper and TransformingReporter which handled compile-time error positions
• //SOURCECODE_ORIGINAL_FILE_PATH and //SOURCECODE_ORIGINAL_CODE_START_MARKER, which handled sourcecode.File/sourcecode.Line positioning

This replaces it with a single ///SOURCE_CODE_START comment referenced by -Ymagic-offset-header:SOURCE_CODE_START, which hooks in early enough in the compiler pipeline that all these downstream use cases can rely on it

The new ///SOURCE_CODE_START comment relies on character offsets unlike line numbers like the previous hacks, and so we needed to change our CodeGen.scala code to avoid mangling lines within the user code block. Concretely, we needed to stop translating object package into abstract class package extends blahblahblah. Instead we translate object package into class _package (which takes up the same number of characters to avoid affecting offsets) and move the extends blahblahblah into the generated code above the ///SOURCE_CODE_START header

[lihaoyi]

Next blocker seems to be that Mill's codegen desugars

object `package` extends mill.Module {

abstract class package_  extends _root_.mill.util.MainRootModule, mill.Module {

This didn't affect the old error position fixup mechanism because it was line based, but the new one is offset based and so the longer line screws up all the subsequent offsets

[lihaoyi]

Seems we might not be able to merge this until Mill 2.x, as the new codegen would require that MainModule be turned into a proper trait (without extending from class RootModule0) in order to make the only-inherit-from-one-class requirement line up.

Fixable, but would break binary compatibility to fix

[lihaoyi]

Got most tests passing with a modified codegen strategy, but will need to wait for Scala 3.8.0 to land with scala/scala3#24301 before we can merge this since that PR will likely miss 3.7.4

[lihaoyi]

Can't land this yet because scala/scala3#24301 hasn't made it into 3.8.0-RC2

[lihaoyi]

Now blocked on scala/scala3#25706