Add \ when adding line breaks in strings in the middle of assignments

[luabud]

Follow up to #6805.
Right now when we add a quotation mark when adding a line break in a long string, we break the code logic when it's around assignments or return statements.

For example:

a = "this is a long| message"  

If we add a line break on a, right now we are making a evaluate to "this is a long":

a = "this is a long"
 "message"  

I believe the ideal would be to do the following:

a = "this is a long" \
 "message"  

Similarly if this was in a return statement.