WIP: Unified IO layer

[SolalPirelli]

Part of #26492

Adds a new IO package (nio, which will eventually be renamed io) and replaces all uses of java.io, java.nio, dotty.tools.io.AbstractFile, and dotty.tools.io.Path in the compiler tests with it (except interacting with the compiler and tests for the old IO stuff)

The new IO package:

• Has File and FileContainer as separate types
  • this ensures one can't call methods that make no sense, e.g., writing to a directory or getting a file's subfiles
  • a directory is a file container, but it's not the only kind, ZIPs and JARs are also containers
  • for ZIPs and JARs one can get a File representing the file and then obtain a FileContainer from that representing its insides (but they cannot be confused)
• Does not expose java.io.File nor java.nio.file.Path at all
• Ensures all operations to read/write text require a Codec
• Supports multi-release JARs, with an API that requires a version always
• Provides convenience methods to read/write as text, as lines, etc. (that subclasses can override if they have a faster way to do it)
• Encapsulates all implementations completely (so the rest of the code never needs to special case on disk files vs in-memory files etc.)

No other changes for now.

Have you relied on LLM-based tools in this contribution?

No

How was the solution tested?

Covered by existing tests (this is a refactoring)

[SolalPirelli]

reminder for myself