Fix #8404 JS backend doesn't handle float->int type conversion (#15950) [backport]

* Fix #8404 JS backend doesn't handle float->int type conversion
* handle conv to uint as cast, discard other cases
* limit to int32, times use int64
* toInt including tyInt64 break times timezones lib, ignore for now
* also affect to vm
* move to tests/misc/t8404.nim

(cherry picked from commit 797cb2e67b)

tests/misc/t8404.nim

@@ -0,0 +1,33 @@
+discard """
+  targets: "c cpp js"
+"""
+template main() =
+  block: # bug #8404
+    # can conv
+    template float2int(T) =
+      var a = -1.0
+      let b = T(a)
+      doAssert b < 0
+      let c = b + 1
+      doAssert c is T
+      doAssert c == 0
+
+    float2int(int8)
+    float2int(int16)
+    float2int(int32)
+    float2int(int64)
+
+  block:
+    # can handle middle conv
+    # `/` can trigger int to float
+    template float2int(T) =
+      let n = T(1 / 256)
+      doAssert n == 0
+
+    float2int(int8)
+    float2int(int16)
+    float2int(int32)
+    # float2int(int64)
+main()
+static:
+  main()