add darknet type declarations

[robingenz]

This PR fixes #736

[UrielCh]

Was implemented in my fork
in dnn.d.ts

/**
 * Reads a network model stored in Darknet model files.
 * 
 * https://docs.opencv.org/4.x/d6/d0f/group__dnn.html#gafde362956af949cce087f3f25c6aff0d
 * 
 * @param cfgPath 	path to the .cfg file with text description of the network architecture. (should be an absolute path)
 * @param modelPath to the .weights file with learned network. (should be an absolute path)
 */
export function readNetFromDarknet(cfgPath: string, modelPath: string): Net;
export function readNetFromDarknetAsync(cfgPath: string, modelPath: string): Promise<Net>;

this code is PR is still valid.

[UrielCh]

please use @u4/opencv-build readNetFromDarknet is available.

This PR can be close.