Add Milesight WS303 mini leak detection sensor

[AkashM707]

Adds the Milesight WS303 mini leak detection sensor, firmware 1.5, reusing the existing Milesight 1.0.3 profiles for AS923, AU915, EU868 and US915.

The codec is Milesight's published decoder and encoder for this product, combined into a single file.

One thing worth flagging for review: both halves define a getValue helper with opposite semantics — the decoder maps a numeric key to a label, the encoder maps a label back to its key. Concatenating them unchanged lets the encoder's definition win through function hoisting, and every decode then returns "unknown". The encoder's helper is renamed to getEncodeValue here; no other changes to Milesight's code.

Tests cover battery, leakage status (normal / leak), device status, and downlink encoding for reboot and report interval.

[brocaar]

@AkashM707 the tests are failing. I believe it is related to these lines:

function Buffer(size) {
  this.buffer = new Array(size);
  this.offset = 0;

  for (var i = 0; i < size; i++) {
    this.buffer[i] = 0;
  }
}

I did a test and replacing these lines with:

class Buffer {
  constructor(size) {
    this.buffer = new Array(size).fill(0);
    this.offset = 0;
  }
}

solved the issue. Could you update this in your pull-request?