Update SF-0007 to version 4

[iCharlesHu]

• v4: Minor updates (mostly name changes):
  • Dropped the executing label from .run().
  • Removed the references to Subprocess.AsyncBytes:
    • Instead, we use opaque type: some AsyncSequence<UInt8, Error>.
    • When typed throws is ready, we can then update to the actual error type.
  • Updated .standardOutput and .standardError properties on Subprocess and CollectedResult to be non-optional (now they use fatalError instead).
    • Rationale: These properties can only be nil in two cases:
      1. The corresponding .output/.error was not set to .redirectToSequence when run() was called.
      2. These properties are accessed multiple times. This is because these AsyncSequences are reading pipes under the hood, and pipes can only be read once.
    • Both cases can't be resolved until the source code is updated; they are therefore considered programming errors.
  • Updated StandardInputWriter to be non-sendable.
  • Renamed PlatformOptions.additionalAttributeConfigurator to Platform.preSpawnAttributeConfigurator; renamed PlatformOptions.additionalFileAttributeConfigurator to Platform.preSpawnFileAttributeConfigurator.
  • Updated all closeWhenDone parameter labels to closeAfterProcessSpawned.
  • Renamed Subprocess.Result to Subprocess.ExecutionResult.
  • Added Codable support to TerminationStatus and ExecutionResult.
  • Renamed TerminationStatus.exit:
    • From .exit to .exited.
    • From .wasUnhandledException to .isUnhandledException.
  • Added two sections under Future Direction: Support Launching Long Running Processes and Process Piping