fix contents of encapsulated strings being lexed

[teatwig]

I encountered a similar issue as in #10 where creating a window rule like on created if app_id is "f.exe" then maximize would lead to the following error:

on created if app_id is "f.exe" then maximize
                        ^ Literal parser error. Text could not be converted to float. text:"f.exe"

This is because the lexer tries to parse the contents of encapsulated literals.

The only issue I can see with the solution is that it might not handle encapsulated character literals properly? Although I couldn't find a place where character literals are used.
Apart from that it doesn't really make sense to try to parse for example "false" as a boolean instead of a window with the title "false".

It might make sense to handle only literals encapsulated with " differently since at least the comments in lexer.hpp imply that it's supposed to be generic and there might eventually be a use case for encapsulated literals that aren't strings.