Warn on unrecognized dub.sdl entries

[vpanteleev-sym]

If I mistype the name of a setting in dub.sdl, Dub silently ignores it.

This generally does not match user intent.

Though it's nice that one effect of this is that dub.sdl files are backwards-compatible, we should at least print a warning if we don't recognize a setting's name.

[Geod24]

Use dub.json ;)

[Geod24]

Jokes aside, the way I wanted to achieve that was to write a Configy backend for SDL (our vendored version of Configy currently uses HEAD so it's "just" writing the backend).

[vpanteleev-sym]

I would back a DIP which allows Dub to read dub.yaml (and soft-deprecate SDL).

Maybe reuse some rationale from https://github.com/WalterBright/documents/blob/master/bitfields.md#rationale ;)

[Geod24]

#3027 ;)