Add getPosAtLineNumber

[HoldYourWaffle]

I have never encountered as many off-by-one-ish errors as I have today...
However, as far as I can tell this function is now working properly 😄

I also changed a comment somewhere to be more clear about what is 1-indexed.

Closes #891.

[Gerrit0]

Did you know about the getPositionOfLineAndCharacter function provided by TypeScript? It will perform significantly better over repeated calls since it caches the line positions, and avoids copying strings.

[dsherret]

Thanks @Gerrit0 for that suggestion! I didn't know about that.

@HoldYourWaffle what do you think about updating this PR to use that API? It might be nice if it caches stuff for performance and reduces allocations. I think it might be 0-indexed, but ts-morph is 1-indexed.

[HoldYourWaffle]

Sounds good! I'll hopefully get to it later today.

Edit: I found ts.getLineAndCharacterOfPosition as well, so StringUtils.getLineNumberAtPos can be replaced too