Support code hot reloading for JavaScript projects (#7362)

* Support code hot reloading for JavaScript projects

* Add some missing JavaScript symbols and APIs

* fix the Travis build

* (review changes) remove the js type from the standard library as it doesn't follow NEP-1

* more additions to the DOM module

* Follow NEP-1 in jsffi; spell 'hot code reloading' correctly

* introduce a jscore module

* Document jscore module.

* readded js type

* Remove the '$' operator that doesn't behave

lib/js/jscore.nim

@@ -0,0 +1,91 @@
+## This module wraps core JavaScript functions.
+##
+## Unless your application has very
+## specific requirements and solely targets JavaScript, you should be using
+## the relevant functions in the ``math``, ``json``, and ``times`` stdlib
+## modules instead.
+
+when not defined(js) and not defined(Nimdoc):
+  {.error: "This module only works on the JavaScript platform".}
+
+type
+  MathLib* = ref object
+  JsonLib* = ref object
+  DateLib* = ref object
+  DateTime* = ref object
+
+var
+  Math* {.importc, nodecl.}: MathLib
+  Date* {.importc, nodecl.}: DateLib
+  JSON* {.importc, nodecl.}: JsonLib
+
+{.push importcpp.}
+
+# Math library
+proc abs*(m: MathLib, a: SomeNumber): SomeNumber
+proc acos*(m: MathLib, a: SomeNumber): float
+proc acosh*(m: MathLib, a: SomeNumber): float
+proc asin*(m: MathLib, a: SomeNumber): float
+proc asinh*(m: MathLib, a: SomeNumber): float
+proc atan*(m: MathLib, a: SomeNumber): float
+proc atan2*(m: MathLib, a: SomeNumber): float
+proc atanh*(m: MathLib, a: SomeNumber): float
+proc cbrt*(m: MathLib, f: SomeReal): SomeReal
+proc ceil*(m: MathLib, f: SomeReal): SomeReal
+proc clz32*(m: MathLib, f: SomeInteger): int
+proc cos*(m: MathLib, a: SomeNumber): float
+proc cosh*(m: MathLib, a: SomeNumber): float
+proc exp*(m: MathLib, a: SomeNumber): float
+proc expm1*(m: MathLib, a: SomeNumber): float
+proc floor*(m: MathLib, f: SomeReal): int
+proc fround*(m: MathLib, f: SomeReal): float32
+proc hypot*(m: MathLib, args: varargs[distinct SomeNumber]): float
+proc imul*(m: MathLib, a, b: int32): int32
+proc log*(m: MathLib, a: SomeNumber): float
+proc log10*(m: MathLib, a: SomeNumber): float
+proc log1p*(m: MathLib, a: SomeNumber): float
+proc log2*(m: MathLib, a: SomeNumber): float
+proc max*(m: MathLib, a, b: SomeNumber): SomeNumber
+proc min*[T: SomeNumber | JsRoot](m: MathLib, a, b: T): T
+proc pow*(m: MathLib, a, b: distinct SomeNumber): float
+proc random*(m: MathLib): float
+proc round*(m: MathLib, f: SomeReal): int
+proc sign*(m: MathLib, f: SomeNumber): int
+proc sin*(m: MathLib, a: SomeNumber): float
+proc sinh*(m: MathLib, a: SomeNumber): float
+proc sqrt*(m: MathLib, f: SomeReal): SomeReal
+proc tan*(m: MathLib, a: SomeNumber): float
+proc tanh*(m: MathLib, a: SomeNumber): float
+proc trunc*(m: MathLib, f: SomeReal): int
+
+# Date library
+proc now*(d: DateLib): int
+proc UTC*(d: DateLib): int
+proc parse*(d: DateLib, s: cstring): int
+
+proc newDate*(): DateTime {.
+  importcpp: "new Date()".}
+
+proc newDate*(date: int|string): DateTime {.
+  importcpp: "new Date(#)".}
+
+proc newDate*(year, month, day, hours, minutes,
+             seconds, milliseconds: int): DateTime {.
+  importcpp: "new Date(#,#,#,#,#,#,#)".}
+
+proc getDay*(d: DateTime): int
+proc getFullYear*(d: DateTime): int
+proc getHours*(d: DateTime): int
+proc getMilliseconds*(d: DateTime): int
+proc getMinutes*(d: DateTime): int
+proc getMonth*(d: DateTime): int
+proc getSeconds*(d: DateTime): int
+proc getYear*(d: DateTime): int
+proc getTime*(d: DateTime): int
+proc toString*(d: DateTime): cstring
+
+#JSON library
+proc stringify*(l: JsonLib, s: JsRoot): cstring
+proc parse*(l: JsonLib, s: cstring): JsRoot
+
+{.pop.}