Expose std_normal_log_qf

[WardBrian]

Closes #1213. For the time being this is just exposed as a "normal" function. As we add more it will make more sense to categorize them into a new suffix type in our distribution list.

Submission Checklist

• [x] Run unit tests
• Documentation
  • [x] If a user-facing facing change was made, the documentation PR is here: @spinkney agreed to write this up https://github.com/stan-dev/docs/pull/589

Release notes

Added quantile functions (inverse-CDFs) std_normal_qf and std_normal_log_qf. std_normal_qf is a synonym for inv_Phi, and std_normal_log_qf is the same but with the input on the log-scale.

Copyright and Licensing

By submitting this pull request, the copyright holder is agreeing to license the submitted work under the BSD 3-clause license (https://opensource.org/licenses/BSD-3-Clause)

[WardBrian]

Expression tests are failing due to std_normal_qf not really existing in Math, see https://github.com/stan-dev/math/pull/2824