Auto-instrument SvelteKit load functions

[Lms24]

While we're waiting on sveltejs/kit#9542 to be accepted/implemented, we need to apply the alternative solution in the meantime, as instructing users to manually wrap all load functions is not an acceptable ask for GA.

To add performance instrumentation (and load-specific error catching), the SvelteKit SDK needs to automatically wrap load functions. We opt to do this at build-time rather than via the wizard for the following reasons:

• A build-time solution will find and wrap (hopefully) all load functions, while users would need to run the wizard every time to wrap newly added load functions
• A build-time solution doesn't need to be revertible, while the wizard should ideally have a revert command. This is especially important as long-term we want to move away from wrapping load functions directly in favour of a handleLoad hook.
• While modifying just two hooks files is fine IMO to do via the wizard, touching potentially a lot more files than these two is by far more intrusive and error-prone.

This is in line with the feedback we received in #6692 RE auto-wrapping load functions.

ref #6692