Add PathFinder.CostMatrix._bits property

[sparr]

Brief Description

Adds the _bits property to PathFinder.CostMatrix

https://github.com/screeps/engine/blob/97c9d12385fed686655c13b09f5f2457dd83a2bf/src/game/path-finder.js#L19

Fixes #248 (which was previously closed without resolution)

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

I let the commit hook do the compile, which I think is what CONTRIBUTING.md suggests.

[ShuP1]

_bits is widely used but officially undocumented. So it should either have a warning or an explicit type cast