Only recombine split -- arguments after new/init command.

Fixes sbt#7507

Previously, passing JVM options like -Xmx1G directly on the command
line would result in an error:

  sbt -v -Xmx1G
  [error] Expected ';'
  [error] -Xmx1G
  [error]       ^

This was because -X options were being passed to sbt as commands
instead of being recognized as JVM arguments.

Changes:
- Added handling for -X options in sbt.bat to pass them to the JVM
- Updated help text to document this feature
- Added integration tests for the new functionality