Added Decr and DecrBy. (String Commands)

[SanHalacogluImproving]

No description provided.

[acarbonetto]

I think this is the comment for incr. But decr should say "Decrements".

Also, this command doesn't take an amount. We should follow what's in the node side, since they have overloaded this function like we have.

    /** Decrements the number stored at `key` by one. If `key` does not exist, it is set to 0 before performing the operation.
     * See https://redis.io/commands/decr/ for details.
     *
     * @param key - The key to decrement its value.
     * @returns the value of `key` after the decrement. An error is raised if `key` contains a value
     * of the wrong type or contains a string that can not be represented as integer.
     */
    public decr(key: string): Promise<number> {
        return this.createWritePromise(createDecr(key));
    }

    /** Decrements the number stored at `key` by `amount`. If `key` does not exist, it is set to 0 before performing the operation.
     * See https://redis.io/commands/decrby/ for details.
     *
     * @param key - The key to decrement its value.
     * @param amount - The amount to decrement.
     * @returns the value of `key` after the decrement. An error is raised if `key` contains a value
     * of the wrong type or contains a string that can not be represented as integer.
     */
    public decrBy(key: string, amount: number): Promise<number> {
        return this.createWritePromise(createDecrBy(key, amount));
    }

[SanHalacogluImproving]

I'll fix the documentation. I don't understand the rest of your comment decr just has 1 argument and that's the key what are you referring to? DecrBy has amount.

[acarbonetto]

The documentation used to mention "amount" for the overloaded method without "amount". All fixed now. You're good.