feat[next]: Prototype concat_where extension

[SF-N]

Instead of writing nested concat_wheres like in:

@field_operator
  def boundary(...):
      return concat_where(
          K == 0, lower, 
          concat_where(K == nlev, upper, interior)
      )

one can now also write:

@field_operator
  def boundary(...):
      return concat_where(
          (K == 0, lower),
          (K == nlev, upper),
          interior
      )

TODOs:

• The concat_where BuiltinFunction currently only accepts 3 args -> allow a variable argument number
• Fix mypy issues