[web_generator] Improve debugging of bin scripts with better stack traces

The stack traces produced by JS code are usually difficult to map/trace to Dart, and since we plan on maybe making this accessible to users as a public package (see #475), we shouldn't place this burden on users, and instead we should make it possible for easy debugging of this library. We can use some of the JS types (`error.stack` if available else use [`Error.captureStackTrace()`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Error/captureStackTrace) to add the trace) to get the stack trace in JS, parse the trace (either ourselves, or there is a popular [`stack-trace`](https://www.npmjs.com/package/stack-trace) library we can use), and then use the `source_map_stack_trace` package to take the trace and the JS source map to produce a Dart-compatible source trace. > Although stack traces are [not yet standardized](https://github.com/tc39/proposal-error-stacks) in JS, they are widely implemented, and that includes V8 (`node`). Although these stack traces are more helpful, they are not perfect and accurate, and while I'll try to improve the traces presented as best as possible, until they show a completely accurate representation of the code, we should add a `--raw-traces` (for lack of better name; name is tentative) flag to allow users to print the raw trace when reporting issues or bugs (or if we/they want to try to debug), which falls back to this normal trace. @srujzs Wanted to know if this is something that would make sense before working on it.