support useVersion:1.6 (#19601)

tests/system/tuse_version16.nim

@@ -0,0 +1,49 @@
+discard """
+  matrix: "--useVersion:1.6"
+"""
+
+{.warning[UnusedImport]: off.}
+
+import std/[
+  # Core:
+  bitops, typetraits, lenientops, macros, volatile,
+
+  # Algorithms:
+  algorithm, sequtils,
+
+  # Collections:
+  critbits, deques, heapqueue, intsets, lists, options, sets,
+  sharedlist, tables,
+
+  # Strings:
+  editdistance, wordwrap, parseutils, ropes,
+  pegs, punycode, strformat, strmisc, strscans, strtabs,
+  strutils, unicode, unidecode,
+
+  # Generic operator system services:
+  os, streams,
+
+  # Math libraries:
+  complex, math, mersenne, random, rationals, stats, sums,
+
+  # Internet protocols:
+  httpcore, mimetypes, uri,
+
+  # Parsers:
+  htmlparser, json, lexbase, parsecfg, parsecsv, parsesql, parsexml,
+
+  # XML processing:
+  xmltree, xmlparser,
+
+  # Generators:
+  htmlgen,
+
+  # Hashing:
+  base64, hashes,
+
+  # Miscellaneous:
+  colors, sugar, varints,
+]
+
+
+doAssert NimVersion == "1.6.100"