fix(message): Update Flag Iota

[jshlbrd]

Description

• Changes the message package's Flag starting number.

Motivation and Context

This guidance may be outdated because the iota starts at 1 without requiring addition. This small program shows that IsControl (1) works as expected (does not start at zero):

package main

import (
	"fmt"

	"github.com/brexhq/substation/v2/message"
)

func main() {
	msg := message.New()
	fmt.Println("msg flag 0, ", msg.HasFlag(0))
	fmt.Println("msg flag 1, ", msg.HasFlag(1))

	ctrl := msg.AsControl()
	fmt.Println("ctrl flag 0, ", ctrl.HasFlag(0))
	fmt.Println("ctrl flag 1, ", ctrl.HasFlag(1))
}

msg flag 0,  false
msg flag 1,  false
ctrl flag 0,  false
ctrl flag 1,  true

How Has This Been Tested?

See program above.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.