Remove dependency on `debug.loadmodule`

[grilme99]

Jest Lua currently depends on an internal Roblox API that requires flipping the FFlagEnableLoadModule flag. This makes adoption more difficult for many developers.

It's not clear how we can actually do this, though. loadmodule is used to:

• Support mocking a module environment (you cannot mock an entire module when using require)
• Not requiring a top-level function to be returned, like with TestEZ

[christopher-buss]

Can this help? See here.

[westurner]

• From jest-roblox 3.10 (2024-10)

  "✨ Added a fallback to use loadstring instead of loadmodule in lower privileged contexts (#392)" https://github.com/Roblox/jest-roblox/blob/master/CHANGELOG.md#3100-2024-10-02

• https://github.com/RadiatedExodus/RunLSC mentions falling back to FiOne or
  Yueliang if loadstring() is not available; but not loadmodule.
• run-in-roblox does not yet support all of the roblox-cli args:
  • --fastFlags.allOnLuau --fastFlags.overrides EnableLoadModule=true
    • https://roblox.github.io/jest-roblox-internal/upgrading-to-jest3#running-tests :

      roblox-cli run --load.model default.project.json --run spec.lua --fastFlags.allOnLuau  --fastFlags.overrides EnableLoadModule=true
      

  • https://github.com/rojo-rbx/run-in-roblox
• FWIW, Vinegar supports an easier way to modify fflags; but run-in-roblox does not yet support Linux

  [studio.fflags]
  FFlagEnableLoadModule=true