Update explainer on stack trace support in JS API

This updates the explainer on
- We add stack trace option to `Exception` class in the JS API
- `Exception` can contain an optional `externref` to carry a stack trace
- `Exception` is not a subclass of JS `Error`

Addresses WebAssembly#183 and WebAssembly#189.

proposals/exception-handling/Exceptions.md

@@ -394,13 +394,6 @@ within `()` after `delegate`s are the label operands in immediate values.
 
 ### JS API
 
-#### Stack traces
-
-When an exception is thrown, the runtime will pop the stack across function
-calls until a corresponding, enclosing try block is found. Some runtimes,
-especially web VMs may also associate a stack trace that can be used to report
-uncaught exceptions. However, the details of this are left to the embedder.
-
 #### Traps
 
 The `catch`/`catch_all` instruction catches exceptions generated by the `throw`
@@ -455,6 +448,21 @@ access to the data fields of a `Exception` if a matching tag is given. This last
 check ensures that without access to a WebAssembly module's exported exception
 tag, the associated data fields cannot be read.
 
+The `Exception` constructor can take an optional `ExceptionOptions` argument,
+which can optionally contain `traceStack` entry. When `traceStack` is `true`,
+web VMs can attach a stack trace string to `Exception.stack` field, as in
+JavaScript's `Error` class. While `Exception` is not a subclass of JavaScript's
+`Error` and it can be used to represent normal control flow constructs,
+`traceStack` field can be set when we use it to represent errors. The format of
+stack trace strings conform to the [WebAssembly stack trace
+conventions](https://webassembly.github.io/spec/web-api/index.html#conventions).
+When `ExceptionOption` is not provided or it does not contain `traceStack`
+entry, `traceStack` is considered `false` by default.
+
+To preserve stack trace info when crossing the JS to Wasm boundary, `Exception`
+can internally contain an optional `externref` value containing a stack trace
+string, which is propagated when caught by `catch` and rethrown by `rethrow`.
+
 More formally, the added interfaces look like the following:
 
 ```WebIDL
@@ -464,11 +472,17 @@ interface Tag {
   TagType type();
 };
 
+[LegacyNamespace=WebAssembly, Exposed=(Window,Worker,Worklet)]
+dictionary ExceptionOptions {
+  optional boolean traceStack;
+};
+
 [LegacyNamespace=WebAssembly, Exposed=(Window,Worker,Worklet)]
 interface Exception {
-  constructor(Tag tag, sequence<any> payload);
+  constructor(Tag tag, sequence<any> payload, optional ExceptionOptions options);
   any getArg(Tag tag, unsigned long index);
   boolean is(Tag tag);
+  readonly attribute (DOMString or undefined) stack;
 };
 ```