fix #14545 windows CI docs (#14590)

* fix #14545 windows paths
* add lib/std/private/strutils2.nim
* remove dependency strutils2 => strutils
* address comment: lib/std/private/strutils2.nim => compiler/strutils2.nim

tests/stdlib/tstrutils2.nim

@@ -0,0 +1,32 @@
+import "$lib/.." / compiler/strutils2
+
+block: # setLen
+  var a = "abc"
+  a.setLen 0
+  a.setLen 3, isInit = false
+  doAssert a[1] == 'b'
+  a.setLen 0
+  a.setLen 3, isInit = true
+  doAssert a[1] == '\0'
+
+block: # forceCopy
+  var a: string
+  a = "foo"
+  shallow(a)
+  var b: string
+  b = a
+  doAssert b[0].addr == a[0].addr
+  var c: string
+  c.forceCopy a
+  doAssert c == a
+  doAssert c[0].addr != a[0].addr
+
+block: # toLowerAscii
+  var a = "fooBAr"
+  a.toLowerAscii
+  doAssert a == "foobar"
+
+block: # dataPointer
+  var a: string
+  discard a.dataPointer
+  # doAssert a.dataPointer == nil # not guaranteed