[Lang] Make `@ti.func` callable from Python-scope, or `@ti.kfunc` if we're afraid of breaking API

Concisely describe the proposed feature It would be convenient to unify these two concepts, both for API clarity and reducing user learning obstacle. This issue will be just a begining of this refactor.

Describe the solution you'd like (if any) We may first add @ti.kfunc as a experimental one, then replace @ti.func when it's mature and battle-tested.

1. When @ti.func called from Python-scope will be translated into a minimal kernel that executes it, and exit.
2. Add @ti.kfunc that create both a @ti.func and @ti.kernel, when called from corresponding scope, call func/kernel.

def kfunc(foo):
  func_foo = ti.func(foo)
  kernel_foo = ti.kernel(foo)
  @functools.wraps(foo)
  def wrapped(*args, **kwargs):
    if impl.inside_kernel():
      func_foo(*args, **kwargs)
    else:
      kernel_foo(*args, **kwargs)

Additional comments @yuanming-hu @k-ye WDYT? Inputs are welcome!