Ensure Auto variables debugger display works reasonably well in migrated debug adapter #1045

Auto was our attempt to mirror what the Visual Studio debugger provides where it lists the variables it thinks are most relevant in the current debug context (stack frame). This worked better when the debugger was a traditional, non-REPL set up. But now that we execute scripts by dot sourcing them this feature is less useful because all script level variables wind up in the global scope along with dozens of other variables. We could make this work better in the dot source case by taking a "snapshot" of global variables when the session first starts, then Auto would show variables not in that original "global variables" snapshot and variables that are but whose values have changed. When it works, it is very useful.