Add get-function function

[corymhall]

This adds a new function to get information on Pulumi functions, similar
to what we have for resources.

[corymhall]

This change is part of the following stack:

• Add registry functions that return raw schema information #21
  • Add get-function function #32 ◀
    • Add list-functions function #33

_{Change managed by git-spice.}