`ExprConcatStrings` tracks whether the expression is an interpolation or not via an obscure boolean called `forceString`. Instead, we rename it to `isInterpolation`. This is a breaking change for the JSON AST representation. Change-Id: I9f89337449b56f6e99a961e21169761f554c9896 Signed-off-by: Raito Bezarius <raito@lix.systems>