Add SetVM and GetVM methods to Debugger for TLA and extCode integration

[vasusheoran]

This PR introduces two new methods, SetVM and GetVM, to the Debugger struct in debugger.go. These methods provide access to the underlying VM instance associated with the debugger:

• SetVM(vm *VM): Sets the VM instance to be used by the debugger. This allows for dynamic configuration or replacement of the VM during the debugging process, enabling the integration of TLA and ext functionality.
• GetVM() *VM: Returns the current VM instance being used by the debugger. This can be useful for inspecting or modifying the VM's state.

The addition of these methods enhances the flexibility and control of the go-jsonnet debugger, making it easier to integrate TLA and ext features.

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[vasusheoran]

@sbarzowski Can you please take a look?