Implement RFC, rename "class" to "extern" locked behind a debug flag - by replacing function pointers

[karl-police]

Implementation for luau-lang/rfcs#131 incase it gets merged

This is variation 1.

Variation 2 is here: #1927

In this variation I replaced the functions. And the feature is locked behind FFlagDebugLuauRenameClassToExtern, it's prefixed with Debug, so it doesn't get enabled by --fflags=true, which allows us to transition to get rid of every existing reference of "class".

The reason why I replace functions in this variation, is because I thought that this is the optimal way, and won't add an extra instruction everytime you'd call :is

[karl-police]

Me when Unit Tests fail here, but not my end.

[karl-police]

If this fails as well, I probably have to use declare class syntax for the Unit Test

[karl-police]

Works, but I think the tenary operator is doing the opposite thing?

This passed the Unit Tests, soo...

[karl-police]

Alright, I did

static std::string getTag_DEPRECATED(lua_State* L, TypeFunctionTypeId ty);
static std::string getTag_NEW(lua_State* L, TypeFunctionTypeId ty);

// Original function is named "getTag"
using GetTagFunc = std::string (*)(lua_State*, TypeFunctionTypeId);
static GetTagFunc getTag = getTag_DEPRECATED;

TypeFunctionRuntime::TypeFunctionRuntime(NotNull<InternalErrorReporter> ice, NotNull<TypeCheckLimits> limits)
    : ice(ice)
    , limits(limits)
    , state(nullptr, dummyStateClose)
{
    if (FFlag::DebugLuauRenameClassToExtern)
    {
        getTag = getTag_NEW;
    }
}

and that worked

[aatxe]

This is not the appropriate way to flag this, the other PR is correct.