Add framework, based on testapp

[Capital-Asterisk]

Heavily addresses #288, most info on these new changes are here.

This can allow the GDExtension application (#294) to not need to depend on testapp.

• Rework "Sessions" into "Features"
• Simplify TaskBuilder
• Allow running Magnum on the main thread, likely fixing crashing issues on mac.
• Separate renderer setup from scenarios. This would likely allow the GDExtension application to use the same scenarios.
• Allow reading CLI commands while the Magnum window application is open.
• Allow changing scenarios while the window is open
• Restructure directories to match GDExtension branch
• Remove a heck ton of a lot of macros and unnecessary boilerplate for for setting up Features/Sessions

[Capital-Asterisk]

almost 10,000 line changes and rising 🐳 🐦...

I wrote a little unit test / tutorial that shows the important parts of using the new framework:
https://github.com/TheOpenSpaceProgram/osp-magnum/blob/5e8a0b5d3cae3aca5412ff0d90814ac481721763/test/framework/main.cpp

A ton of grunt work required to rewrite all the setup_* 'Session' functions into 'Features'. This gets rid of a bunch of macros and janky stuff and shrinks down the code a tiny bit:

Before and after for 'bounds' (click to expand)

Before:

// Header 1
#define TESTAPP_DATA_BOUNDS 2, \
    idBounds, idOutOfBounds
struct PlBounds
{
    PipelineDef<EStgCont> boundsSet         {"boundsSet"};
    PipelineDef<EStgRevd> outOfBounds       {"outOfBounds"};
};

// Header 2
osp::Session setup_bounds(
        osp::TopTaskBuilder&        rBuilder,
        osp::ArrayView<entt::any>   topData,
        osp::Session const&         scene,
        osp::Session const&         commonScene,
        osp::Session const&         physShapes);

// Source
Session setup_bounds(
        TopTaskBuilder&             rBuilder,
        ArrayView<entt::any> const  topData,
        Session const&              scene,
        Session const&              commonScene,
        Session const&              physShapes)
{
    OSP_DECLARE_GET_DATA_IDS(commonScene,   TESTAPP_DATA_COMMON_SCENE);
    OSP_DECLARE_GET_DATA_IDS(physShapes,    TESTAPP_DATA_PHYS_SHAPES);
    auto const tgScn    = scene         .get_pipelines<PlScene>();
    auto const tgCS     = commonScene   .get_pipelines<PlCommonScene>();
    auto const tgShSp   = physShapes    .get_pipelines<PlPhysShapes>();

    Session out;
    OSP_DECLARE_CREATE_DATA_IDS(out, topData, TESTAPP_DATA_BOUNDS);
    auto const tgBnds = out.create_pipelines<PlBounds>(rBuilder);

    rBuilder.pipeline(tgBnds.boundsSet)     .parent(tgScn.update);
    rBuilder.pipeline(tgBnds.outOfBounds)   .parent(tgScn.update);

    top_emplace< ActiveEntSet_t >       (topData, idBounds);
    top_emplace< ActiveEntVec_t >       (topData, idOutOfBounds);

    rBuilder.task()
        .name       ("Check for out-of-bounds entities")
        .run_on     ({tgScn.update(Run)})
        .sync_with  ({tgCS.transform(Ready), tgBnds.boundsSet(Ready), tgBnds.outOfBounds(Modify__)})
        .push_to    (out.m_tasks)
        .args       ({            idBasic,                      idBounds,                idOutOfBounds })
        .func([] (ACtxBasic const& rBasic, ActiveEntSet_t const& rBounds, ActiveEntVec_t& rOutOfBounds) noexcept
    {
...

After:

// Header 1
struct FIBounds {
    struct DataIds {
        DataId bounds;
        DataId outOfBounds;
    };

    struct Pipelines {
        PipelineDef<EStgCont> boundsSet         {"boundsSet"};
        PipelineDef<EStgRevd> outOfBounds       {"outOfBounds"};
    };
};

// Header 2
extern osp::fw::FeatureDef const ftrBounds;

// Source
FeatureDef const ftrBounds = feature_def("Bounds", [] (
        FeatureBuilder              &rFB,
        Implement<FIBounds>         bounds,
        DependOn<FIScene>           scn,
        DependOn<FICommonScene>     comScn,
        DependOn<FIPhysShapes>      physShapes)
{
    rFB.pipeline(bounds.pl.boundsSet)     .parent(scn.pl.update);
    rFB.pipeline(bounds.pl.outOfBounds)   .parent(scn.pl.update);

    rFB.data_emplace< ActiveEntSet_t >       (bounds.di.bounds);
    rFB.data_emplace< ActiveEntVec_t >       (bounds.di.outOfBounds);

    rFB.task()
        .name       ("Check for out-of-bounds entities")
        .run_on     ({scn.pl.update(Run)})
        .sync_with  ({comScn.pl.transform(Ready), bounds.pl.boundsSet(Ready), bounds.pl.outOfBounds(Modify__)})
        .args       ({    comScn.di.basic,              bounds.di.bounds,        bounds.di.outOfBounds })
        .func([] (ACtxBasic const &rBasic, ActiveEntSet_t const &rBounds, ActiveEntVec_t &rOutOfBounds) noexcept
    {
...
</details>