SendParameters: Constant for mode: 0

[Esorat]

We can define a constant for the 0 mode value to improve readability and describe the default behavior more clearly. This constant will replace any direct usage of 0 in the mode field throughout the codebase, as commonly seen in real-world contracts.
This change will make choosing the default mode more explicit and could be leveraged by SuspiciousMessageMode, which detects such cases: nowarp/misti#199.

Context: https://docs.tact-lang.org/book/message-mode/

[anton-trunov]

The mode field has 0 as the default value, so people often don't even mention it, like code and data if they don't want to deploy a contract.

struct SendParameters {
    bounce: Bool = true;
    to: Address;
    value: Int;
    mode: Int = 0;
    body: Cell? = null;
    code: Cell? = null;
    data: Cell? = null;
}

[jubnzv]

@anton-trunov We have seen many cases in real-world contracts where people simply write mode: 0. It seems they are either trying to explicitly specify the mode or are unsure about the default value.

Should we just ignore these cases or raise warnings asking them to remove the mode parameter?

[anton-trunov]

We can add const SendDefaultMode: Int = 0; (or SendDefaultMode) to send.tact for the sake of completeness