Check for unit constructor validity

[GlennFolker]

Add sanity checks in UnitType's constructor:

• If constructor == null, point them to common Vanilla unit classes.
• If constructor != null but EntityMapping.map(name) returns null, register the unit type's name to nameMap.
• If idMap[constructor.classId()] == null, throw an exception.
• If constructor.classId() != idMap[constructor.classId()].get().classId(), throw an exception.
• Otherwise, all behaviors are deemed correct and won't cause confusing bugs after IO.

PR preliminary checks

• I have read the contribution guidelines.
• I have ensured that my code compiles, if applicable.
• I have ensured that any new features in this PR function correctly in-game, if applicable.

[nullotte]

una empanada