This commit removes commandline_background.ts. I believe this is useful
because the only thing it did was provide recvExStr, which just
triggered a synthetic "onLine" event the only consumer of which was the
parser. Since we already used controller_background + acceptExCmd in
some places, it made sense to me to directly use controller_background +
acceptExCmd everywhere.
All functions in native_background.ts use browserBg in order to interact
with the native messenger. This means that these functions can also be
used in the content script. This means that there's no point in keeping
these functions in the background/ folder and that there's no point in
having a native_background message type.
