Java: Make `JumpStmt` a proper superclass

[Marcono1234]

Currently CodeQL's BreakStmt, YieldStmt and ContinueStmt have no common superclass, and JumpStmt is defined separately. This has the following disadvantages:

• It makes it difficult for users to discover JumpStmt because at least in the documentation there is no relation between for example BreakStmt and JumpStmt
• The predicates for accessing the targeted element (a pretty common use case) are only defined for JumpStmt. This requires double casts to access these predicates, multiple of these can be seen in the CodeQL source code, e.g. exit.(BreakStmt).(JumpStmt).getTarget()

What do you think about making JumpStmt a proper superclass of these classes? This would solve the aforementioned issues.

[smowton]

Seems sensible to me, PR welcome :)