fix(js/ts): make Exception.ToString() return the message instead of "Exception"

[klofberg]

Fix: Exception.ToString() returns "Exception" instead of the message (JS/TS)

Problem

Since v2, calling ToString() on a caught exception prints the literal string "Exception" instead of the message:

try
    failwith "boom"
with ex ->
    Browser.Dom.console.log(ex.ToString())
// v1: "boom"  →  v2: "Exception"

Cause

PR #4197 ("[JS/TS] Replace Error with Exception") changed System.Exception so it compiles to Fable's own Exception class (in Util.ts) rather than JavaScript's native Error. That class — intentionally not derived from Error for performance (#2160) — has no toString() method, so ex.ToString() (routed through Types.toString) falls back to returning the constructor name. Previously, native Error.prototype.toString() returned "<name>: <message>", so the message was visible.

Fix

Add a toString() to the Exception class returning this.message. This matches the Dart (ExceptionBase.toString() => this.message) and Python (native __str__) targets, which were already correct. F# exception declarations (FSharpException) keep their own record-formatting toString(), and the BCL exception subclasses in System.fs inherit the new behavior.

Scoped to the JS/TS runtime library only; no compiler changes needed.

Testing

• Added a regression test in tests/Js/Main/TypeTests.fs asserting ex.ToString() contains the message (passes on both .NET and Fable).
• Verified via quicktest (ex.ToString() now prints boom) and the full JS test suite (no regressions).

[MangelMaxime]

Replaced by #4635

I could not push to you fork, so I needed to make a new PR.

Thank you for the fix