move dollars out of system

• the compiler keeps bootstrapping with -d:nimPreviewSlimSystem
• the std/dollars uses new float algorithm when -d:nimPreviewSlimSystem is enabled, otherwise you can still switch them on/off by nimPreviewFloatRoundtrip

ref https://github.com/nim-lang/Nim/pull/19388 ref https://github.com/nim-lang/RFCs/issues/437

Was this discussed previously? Doesn't echo need $?

Edit: Apparently it was discussed in #19388 but just for floats. dollars has way more than that, plus addFloat is still included in system here, so compilation time isn't saved. I think this is an overly dramatic thing to do just to enable roundtrip float-string conversion. If it was for the sake of separating $, then I would understand it, but $ is deeply integrated in a lot of Nim code so it's not going to be easy to just casually do.