ESM support

[kaushals-mecha]

Currently the package works as expected in commonjs module
but with esm, we have to use it like below

import plot from 'simple-ascii-chart';

plot.default(            // plot.default instead of plot
        [
          [1, 1],
          [2, 4],
          [3, 4],
          [4, 2],
          [5, -1],
          [6, 3],
          [7, -1],
          [8, 9],
        ],
        { title: 'Important data', width: 20, height: 8 }
      )
    );