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/impure/db_sqlite.nim

@@ -172,6 +172,8 @@ import db_common
 export db_common
 
 import std/private/[since, dbutils]
+when defined(nimPreviewSlimSystem):
+  import std/assertions
 
 type
   DbConn* = PSqlite3  ## Encapsulates a database connection.

lib/impure/nre.nim

@@ -66,6 +66,9 @@ from strutils import `%`
 import options
 from unicode import runeLenAt
 
+when defined(nimPreviewSlimSystem):
+  import std/assertions
+
 export options
 
 type

lib/impure/re.nim

@@ -34,6 +34,9 @@ runnableExamples:
 import
   pcre, strutils, rtarrays
 
+when defined(nimPreviewSlimSystem):
+  import std/syncio
+
 const
   MaxSubpatterns* = 20
     ## defines the maximum number of subpatterns that can be captured.