Improve expo analytical integral stability

[swetank18]

Motivation

The analytical integral of exponential TF1 functions computes differences of
exponentials, which can suffer from numerical cancellation or overflow when the
integration bounds are close or the exponent is large.

Changes

• Replace the direct difference of exponentials with a numerically stable
  expm1-based formulation

Impact

• Improves numerical robustness without changing the mathematical result
• No API changes

[hageboeck]

This will clash with #20992. Should we close that one?