Strict return types

[tiennou]

Brief Description

This should go after #266 (yep, I mixed up by pushing it later, my bad).

The plan is to remove the crutch of ScreepsReturnCode, CreepActionReturnCode and CreepMoveReturnCode because there's a few places where they're too "wide". So I reviewed all of the docs (and even did some runtime spelunking in cases that were ambiguous). At the end of this, only CreepMoveReturnCode remains in use for Creep (though it could be dropped entirely from the signatures if that's a direction people like), as it can't be used for PowerCreep because of ERR_TIRED.

Checklists

• Test passed
• Coding style (indentation, etc)
• Edits have been made to src/ files not index.d.ts
• Run npm run compile to update index.d.ts

[tiennou]

That caught a few missing things as well. I guess those could be extracted if the whole idea feels like an issue.