$ now works for unsigned intergers with nim js (#14122)

* $(unsigned) now works for js
* move NimMajor+friends closer to NimVersion according as per reviewer feedback

tests/system/tdollars.nim

@@ -0,0 +1,72 @@
+discard """
+  targets: "c cpp js"
+"""
+
+#[
+if https://github.com/nim-lang/Nim/pull/14043 is merged (or at least its
+tests/system/tostring.nim diff subset), merge
+tests/system/tostring.nim into this file, named after dollars.nim
+
+The goal is to increase test coverage across backends while minimizing test code
+duplication (which always results in weaker test coverage in practice).
+]#
+
+import std/unittest
+template test[T](a: T, expected: string) =
+  check $a == expected
+  var b = a
+  check $b == expected
+  static:
+    doAssert $a == expected
+
+template testType(T: typedesc) =
+  when T is bool:
+    test true, "true"
+    test false, "false"
+  elif T is char:
+    test char, "\0"
+    test char.high, static($T.high)
+  else:
+    test T.default, "0"
+    test 1.T, "1"
+    test T.low, static($T.low)
+    test T.high, static($T.high)
+
+block: # `$`(SomeInteger)
+  # direct tests
+  check $0'u8 == "0"
+  check $255'u8 == "255"
+  check $(-127'i8) == "-127"
+
+  # known limitation: Error: number out of range: '128'i8',
+  # see https://github.com/timotheecour/Nim/issues/125
+  # check $(-128'i8) == "-128"
+
+  check $int8.low == "-128"
+  check $int8(-128) == "-128"
+  when not defined js: # pending https://github.com/nim-lang/Nim/issues/14127
+    check $cast[int8](-128) == "-128"
+
+  var a = 12345'u16
+  check $a == "12345"
+  check $12345678'u64 == "12345678"
+  check $12345678'i64 == "12345678"
+  check $(-12345678'i64) == "-12345678"
+
+  # systematic tests
+  testType uint8
+  testType uint16
+  testType uint32
+  testType uint
+
+  testType int8
+  testType int16
+  testType int32
+
+  testType int
+  testType bool
+
+  when not defined(js): # requires BigInt support
+    testType uint64
+    testType int64
+    testType BiggestInt