Fix return types

[tiennou]

Brief Description

Related to #270, but actually state everything's return value explicitly.

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]

Fixed the missing ERR_NOT_OWNER on RoomPosition.createConstructionSite().

[tiennou]

I had to touch the test file (because of .lookAt() returning ERR_INVALID_ARGS when passed out of bounds coordinates), but npm run test didn't catch it (I found it because TS in my OM workspace saw it).

Maybe something to consider; the testsuite doesn't validate that TS is happy with the screeps-test.ts file

[tiennou]

Updated with the bits of #270 that were incorrectly there (the Creep and PowerCreep method returns)