Re-merge AmmonitePhase and Preprocessor into scala-3 from scala-3.4.2+ etc

[Ichoran]

These can be written in ways that are not compiler-specific.

Compiler, DirectoryClassPath, and SyntaxHighlighting contain changes to the compiler internals, including some type signature changes, so they remain separate.

Specifically, Compiler needs an extra parameter for CompilationUnit and ExtractSemanticDB needs an operation specified; DirectoryClassPath has to work with BinaryFileEntry since ClassFileEntryImpl is private; and in SyntaxHighlighting, commentSpans became comment.span. (The last of these changes was 3.3.1 to 3.3.2+; the others 3.3.x to 3.4.2+, and we're skipping 3.4.0 and 3.4.1 because they don't play well with $ as an object.