Add back support for de-emphasized stack frames (smart step) as opt-in feature via settings

[rkeithhill]

We used to examine the source path of every stack frame and if it wasn't under the current working dir, we would show it as deemphasized. That behavior has been removed. But we would like to add in opt-in settings to re-enable this feature. The settings (or debug config) might look something like this:

"smartStep": {
    enabled: true,
    sourceDirs: [ "${workspaceFolder}" ]
}