followup #18318: simplify dollarImpl and add a test (#18330)

tests/stdlib/tdigitsutils.nim

@@ -0,0 +1,23 @@
+import std/private/digitsutils
+
+template main =
+  block: # digits10
+    doAssert digits10(0'u64) == 1
+    # checks correctness on all powers of 10 + [0,-1,1]
+    var x = 1'u64
+    var num = 1
+    while true:
+      # echo (x, num)
+      doAssert digits10(x) == num
+      doAssert digits10(x+1) == num
+      if x > 1:
+        doAssert digits10(x-1) == num - 1
+      num += 1
+      let xOld = x
+      x *= 10
+      if x < xOld:
+        # wrap-around
+        break
+
+static: main()
+main()