Cannot build with neopixels extension

[martinwork]

Describe the bug

If the Neopixels extension is added with PlanetX, either by the user or as dependency of another extension, the project cannot build.

There are muliple duplicate definition errors in the browser console.

This was reported to https://support.microbit.org/ (ticket number 61588) by a user trying to use these extensions together
https://github.com/elecfreaks/pxt-PlanetX
https://github.com/elecfreaks/pxt-wukong

For a potential fix, please see the discussion on this PR: microsoft/pxt-microbit#4946

To Reproduce
Create new project. Add PlanetX. Add Neopixels or wuKong. Download. See error.

Expected behavior
Extensions can be used together.