Add optional flag (?) to overloads in love2d and lovr builders

[C3pa]

Summary of changes:

• Made builders add ? next to argument names if the argument has a default value.
• isTableOptional refactored to getOptional that returns '?' or an empty string if based on whether the argument is or is not optional. Ready to be inserted in a string with string.format.

The second change is a separate commit and can be reverted if not desired.

[sumneko]

Thank you!