sceneInterpolator: Ensure unescaped backslashes are escaped

[kaydelaney]

Should fix grafana/grafana#101037

Changes the old _interpolateVariablesInTransformationConfigs behavior to deeply interpolate over strings in the object (hierarchy) instead of stringifying -> interpolating -> parsing.

[mdvictor]

I think this fixes the escaping issue, but doesn't fully fix grafana/grafana#101037. The dashboard is still broken because of that panel although the error is now different

[kaydelaney]

Thanks for taking a look @mdvictor! ThedeepIterate function I wrote was mutating the passed object directly before, which was causing a test failure (and maybe the reason the dashboard was still broken? I haven't checked yet 🫣).
Let me know if it's working better now :)

[leeoniya]

please look at grafana/grafana#101438 (comment) and make sure this scenes variant works with the dashboard attached in that PR description.