syscall/js: support for Go 1.14 changes

[dmitshur]

There will be changes to the syscall/js API in Go 1.14:

• JavaScript values referenced from Go via js.Value objects can now be garbage collected.
• js.Value values can no longer be compared using the == operator, and instead must be compared using their Equal method.
• js.Value now has IsUndefined, IsNull, and IsNaN methods.

We should make it so that when people update their Go code to target syscall/js API in Go 1.14, it can continue to build with both GOARCH=wasm (Go WebAssembly) and GOARCH=js (GopherJS).

I think this can be done in a backwards-compatible way by adding the Equal, IsUndefined, IsNull, and IsNaN methods to js.Value.

/cc @hajimehoshi

[visualfc]

https://github.com/visualfc/gopherjs/tree/go1.14/compiler/natives/src/syscall/js

implement and test go1.14 js

[mvdan]

I just got bit by this too; I was really confused at js.Value.IsUndefined not being defined for a while.