For now, when we pause, the debugger frontend would always force selecting the original source. That, even if we were previously selecting a generated file.
This goes against the overall goal being done elsewhere to "keep context", and select the same type of source as the currently selected one (original if original is selected, or generatd otherwise).

We could still try to open the original source if no source is currently selected,
but when a source is already selected, try to keep the same context.

Instead, if we were selecting a generated source, we would keep selecting generated source.
If there is no selected source, we would still try to select the original source.

Many tests were relying on this behavior.

WASM test was failing because of errors with the parser worker.
I added early bailout in order to prevent trying to use the parser worker for WASM.

shared-head test helper were also tweaked to better support select WASM binary sources.

The worker itself ignores already the WASM sources,
but we could save some cycle to avoid calling the parserWorker entirely
when we know the source won't be supported by it.

https://hg.mozilla.org/mozilla-central/rev/81fb6fc69cca
https://hg.mozilla.org/mozilla-central/rev/bb12f557adeb

Alongside bug 1634721 this improved stepping through original files and regular js modules. You context will be better preserved.