Refactor Jvm methods to match specified signatures

[vishwamartur]

Related to #3772

Refactor Jvm.scala to consolidate subprocess and classloader spawning operations into four specified signatures.

• Refactor callSubprocess method:

  • Rename to call.
  • Update parameters to match the specified call signature.
  • Use jvmCommandArgs to generate command arguments.
  • Call os.call with the updated parameters.

• Refactor runSubprocess method:

  • Rename to spawn.
  • Update parameters to match the specified spawn signature.
  • Use jvmCommandArgs to generate command arguments.
  • Call os.spawn with the updated parameters.

• Add spawnClassloader method:

  • Create a new method to match the specified spawnClassloader signature.
  • Use mill.api.ClassLoader.create to create a classloader.

• Add callClassloader method:

  • Create a new method to match the specified callClassloader signature.
  • Use spawnClassloader to create a classloader and set it as the context classloader.
  • Execute the provided function with the new classloader and restore the old classloader afterward.

• Add tests in JvmTests.scala:

  • Add tests for the new call method.
  • Add tests for the new spawn method.
  • Add tests for the new callClassloader method.
  • Add tests for the new spawnClassloader method.