Make LineLens.configChangedHandler update the config directly, instead of through a ref

[Numpsy]

WHAT

Not sure if this is the right thing to do, but:

I was looking at #1924 about the missing leading space in the lens text.
Whilst testing, I tried changing the user setting for the lens prefix from ' // ' to a custom value (e.g. ' ** ') and noticed that that wasn't picked up either.

It looks to me like the intent is for the 'default' value (the one changed in #1928) to be overwritten be a configuration specifed value that gets loaded in the configChangedHandler call made from the activate event.
However - when I tried debugging it it looked like the updated prefix value wasn't peristed into the config member of LineLens - it looked to always be using the local default.

I'm not sure what's causing that to go wrong, but - I'm not sure why the config object is passed into configChangedHandler by reference and from a bit of testing it seems that just making it update the config member directly causes the whole thing to correctly pick up a user specified prefix string.

So - here it is for any comments from anyone who knows more about how it's supposed to work.

WHY

refs #1924

HOW

Update the config member of LineLens directly instead of through a reference.

[TheAngryByrd]

🎸 Thanks!!!