Allow hiding deprecated config options

[peterebden]

This marks all the language-specific stuff as deprecated and adds a feature flag that stops asp from getting them on its config object.

Hopefully this will start cutting down on errors from these (e.g. #2851 ran across another one that we never updated and which had been silently using the wrong Go tool for months); we should find out immediately that these don't exist.

[stale]

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[peterebden]

Hush, bot.

I'm not sure what the immediate future is here - the main issue is that I'm not sure how to update the C++ plugin to work in the new world as it did before. We need to figure that out anyway though.
Maybe I should also try to split out some of the noncontroversial bits from here (although to me their existence argues for the utility of this feature, so I would like to get it sorted sometime).