chore(measure_theory/special_functions/basic): split (#19040)
Split out the facts about `is_R_or_C` from `measure_theory/function/special_functions/basic` (a foundational file, imported by the Bochner integral construction). These facts are heavier-weight than one would expect because the fact that an `is_R_or_C` field is a proper space currently passes through the corresponding fact for a general finite-dimensional normed space.
