python: Add registry configuration

[idbrii]

• Closes install python with windows registry #128

PEP 514 specifies how python should be added to the registry so
third-party installers, binary packages depending on python (vim), and
pre-compiled binary extension packages (kivy, Pillow, PyGame) can find
python.

I think it also helps resolve python finding 32-bit vs 64-bit
executables.

Seems that python requires the "PythonCore" key. Testing with gvim:
gvim +"py3 print(3)"
crashes with "Scoop" but works with "PythonCore". We're installing
python built by the Python Software Foundation, so it might be matching
that name somewhere inside of python.exe.

Since the manifest uses github urls, it won't work until merged into master. If you want to check it out, I made a testing branch that points to itself.

[idbrii]

Works great on my testing branch:

> scoop uninstall python & scoop install python
Uninstalling 'python' (3.8.2).
'python' was uninstalled.
Installing 'python' (3.8.2) [64bit]
Loading install-pep-514.reg from cache
Checking hash of install-pep-514.reg ... ok.
Loading uninstall-pep-514.reg from cache
Checking hash of uninstall-pep-514.reg ... ok.
Loading python-3.8.2-amd64.exe from cache
Checking hash of python-3.8.2-amd64.exe ... ok.
Running pre-install script...
Running installer script...
Linking ~\scoop\apps\python\current => ~\scoop\apps\python\3.8.2
Creating shim for 'python'.
Creating shim for 'pythonw'.
Creating shim for 'python3'.
Creating shim for 'idle'.
Creating shim for 'idle3'.
'python' (3.8.2) was installed successfully!
Notes
-----
Allow applications and third-party installers to find python by running: C:\Users\idbrii\scoop\apps\python\current\install-pep-514.reg

> gvim +"py3 print(3)"
# 3 is printed to gvim's message area.