text: Title: Add //flutter/fml/math.h with common math constants.
  Body: This came up recently with the use of `cmath` on Windows requiring `_USE_MATH_DEFINES` to be defined before common constants could be accessed.
  
  This define either needs to define at the toolchain level or wired up in the private configs for the targets that use it or the public configs of the target that specify it in their headers.
  
  This platform specific oddity build oddity is hard for developers to reason about. We just just define an use our own constants. Something like would work well. Additional math utilities may be added here.
  
  ```
  // e
  constexpr double kE = 2.7182818284590452354;
  // log_2 e
  constexpr double kLog2_E = 1.4426950408889634074;
  // log_10 e
  constexpr double kLog10_E = 0.43429448190325182765;
  // log_e 2
  constexpr double klogE_2 = 0.69314718055994530942;
  // log_e 10
  constexpr double klogE_10 = 2.30258509299404568402;
  // pi
  constexpr double kPi = 3.14159265358979323846;
  // pi/2
  constexpr double kPiOver2 = 1.57079632679489661923;
  // pi/4
  constexpr double kPiOver4 = 0.78539816339744830962;
  // 1/pi
  constexpr double k1OverPi = 0.31830988618379067154;
  // 2/pi
  constexpr double k2OverPi = 0.63661977236758134308;
  // 2/sqrt(pi)
  constexpr double k2OverSqrtPi = 1.12837916709551257390;
  // sqrt(2)
  constexpr double kSqrt2 = 1.41421356237309504880;
  // 1/sqrt(2)
  constexpr double k1OverSqrt2 = 0.70710678118654752440;
  ```