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

lib/system/reprjs.nim

@@ -8,6 +8,9 @@
 #
 # The generic ``repr`` procedure for the javascript backend.
 
+when defined(nimPreviewSlimSystem):
+  import std/formatfloat
+
 proc reprInt(x: int64): string {.compilerproc.} = $x
 proc reprFloat(x: float): string {.compilerproc.} = $x