Add ChangeLevel native & OnMapChange forward

[nulxrd]

Description

Add ChangeLevel native & OnMapChange forward

Motivation and Context

I added the native and forward so that it was possible correctly to change the map

How has this been tested?

public void OnClientPutInServer()
{
    ChangeLevel("cs_militia");
}
public PluginReturn OnMapChange(const char[] map) {
    printToServer("map %s", map);
    return PluginStop;
}

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:

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