Include stack in RuntimeError

[wyozi]

I think including stack in errors is definitely a good thing, but

• Should it be in other ErrorKinds than RuntimeError? Probably not since it might not really be feasible to even get the stack for those.
• Should the stack just be blindly appended in fmt?
• This breaks backward compat due to new struct item, but user code shouldn't even use ErrorKind::RuntimeError
• It would be amazing if the stack of the caller of Rust code that errors with Error::External would be recorded in a stack somewhere
  • If we want at least Rust location to be in there, #[track_caller] might be useful (stabilizes in 1.46 https://github.com/rust-lang/rust/pull/72445/files#diff-23e04906de6f359a78328a93305eca79R228)
  • Otherwise it's probably possible to get the stack trace from https://duktape.org/api.html#duk_safe_to_stacktrace and add it to the error in push_error, although this requires a duk-sys update as well

The whole output of fmt looks like this:

nope: JavaScript runtime error (TypeError): TypeError: nope
    at woah (file:1)
    at eval (file:1) preventsyield