get the shape of a 2d array as a 2-tuple describing the major-axis's length, and the minor-axis's length.

import { assertEquals } from "jsr:@std/assert"

const arr2d: Array2DRowMajor<number> = [
[1 , 2 , 3 , 4 , 5 ],
[6 , 7 , 8 , 9 , 10],
[11, 12, 13, 14, 15],
]
const [rows, cols] = shapeOfArray2D(arr2d)
assertEquals(rows, 3)
assertEquals(cols, 5)