initial messages support

[Mistrick]

copy from amxmodx

Description

Motivation and Context

How has this been tested?

#include <spmod>

public PluginInfo pluginInfo =
{
    name = "test2",
    version = "0.0.0",
    author = "author",
    url = "https://github.com/Amaroq7/SPMod"
};

public void OnPluginInit()
{
    int SayText = GetUserMsgId("SayText");
    printToServer("Message SayText = %d\n", SayText);
    char msgname[32];
    GetUserMsgName(SayText, msgname, sizeof msgname - 1);
    printToServer("Message %d = %s\n", SayText, msgname);
}

public void OnClientPutInServer(int client)
{
    int SayText = GetUserMsgId("SayText");
    MessageBegin(MSG_ALL, SayText, {0, 0, 0});
    WriteByte(client);
    WriteString("\1Test\3Hello");
    MessageEnd();
}

Other cases difficult to test without own cmd registration

Screenshots (if appropriate):

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

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

[Amaroq7]

LGTM. Thanks!