Shared typeclass instances for ℝ-scalar-on-ℂ

In mathlib 4.29, NormSMulClass.toIsBoundedSMul is no longer an instance (to avoid loops), which breaks the chain

NormedSpace ℝ ℂ → NormSMulClass ℝ ℂ → IsBoundedSMul ℝ ℂ → ContinuousSMul ℝ ℂ

We provide all three instances here so every file in the project can import LeanModularForms.ForMathlib.Instances instead of redeclaring them.

We also provide IsScalarTower ℝ ℂ ℂ which was previously redeclared in several files with different proof terms.