make more standard libraries work with nimPreviewSlimSystem (#20343)

* make more standard libraries work with `nimPreviewSlimSystem`

* typo

* part two

* Delete specutils.nim

* fixes more tests

* more fixes

* fixes tests

* fixes three more tests

* add formatfloat import

* fix

* last

tests/stdlib/tjsonutils.nim

@@ -7,6 +7,7 @@ import std/json
 from std/math import isNaN, signbit
 from std/fenv import epsilon
 from stdtest/testutils import whenRuntimeJs
+import std/[assertions, objectdollar]
 
 proc testRoundtrip[T](t: T, expected: string) =
   # checks that `T => json => T2 => json2` is such that json2 = json