Get node at (line, col)

**Is your feature request related to a problem? Please describe.**
I get line & column coordinates from an external source, I want to get the node(s) at this coordinate.

**Describe the solution you'd like**
The obvious solution would be `sourceFile.getChildAtPos`, but this takes a 1D coordinate. I can convert from "pos" to line+col using `sourceFile.getLineAndColumnAtPos` (#801) but I can't find anything for the reverse.

**Describe alternatives you've considered**
I bodged together this:
```ts
for (const node of source.getDescendants()) {
if (line === node.getStartLineNumber() && col === node.getStart() - node.getStartLinePos()) {
// We have our node!
}
}
```
But that doesn't seem like an efficient solution.