Activate features per source file

[marler8997]

Provide a way to activate new features. We could piggyback off the version feature for this. First we need to know if the "activate new features" feature is supported. We can do this by defining a version in the compiler when it is supported, i.e.

version (CanActivateFeatures)
{
    
}

If your source code requires new features, you could so something like

version (CanActivateFeatures)
{
    //...
}
else static assert(0, "This code does not work with this compiler, it does not support activating new features");

Then to activate new features, you could use versions as well

version (CanActivateFeatures
{
    version = EnableInterpolatedStrings;
}

You can also do this:

version (CanActivateFeatures
{
    version (SupportsInterpolatedStrings)
    {
        version = EnableInterpolatedStrings;
    }
    else static assert (0, "This code requires InterpolatedString support");
}

[wilzbach]

Non-complete list of things which this flag should activate.

• Classes should be final by default #39 (classes should be final by default)
• Remove Object.factory #36 (remove Object.factory)
• Make @safe the default #32 (@safe by default)
• Deprecate/remove imaginary and complex types #22 (no imaginary & complex types)
  ...

[ntrel]

scope should be the default for parameters, and have e.g. @escape to mark escaping parameters.