Function getClosestIntegerKey

getClosestIntegerKey(data: ReadonlyMap<number, any>, target: number): number
Gets the closest integer key to target in data.
- Requires map to have numbers as keys, not strings
- Math.round is used for rounding target.
Examples:
```
// Assuming numeric keys 1, 2, 3, 4 exist:
getClosestIntegerKey(map, 3);    // 3
getClosestIntegerKey(map, 3.1);  // 3
getClosestIntegerKey(map, 3.5);  // 4
getClosestIntegerKey(map, 3.6);  // 4
getClosestIntegerKey(map, 100);  // 4
getClosestIntegerKey(map, -100); // 1
```
Parameters
- data: ReadonlyMap<number, any>
  Map
- target: number
  Target value
Returns number