Fix v3 compatibilities

[zsu]

Changes to fix v3 compatibilities. 'override' is renamed to 'overrideExisting' since override is a function in v3.

[seanhamlin]

Actual lifesaver. This PR works great.

[m08pvv]

When this PR will be merged? It would be awesome to have it in the npm package

[igorlukanin]

@nagix Any chance you can take a look at this one, merge, and release it? I'd like to mention it as a Chart.js v3-compatible, working one in Chart.js docs on colors: chartjs/Chart.js#10764 (comment)

[homestar9]

@zsu excellent work with the fixes for V3!
@m08pvv, I appreciate you pushing up your NPM package! I mentioned it in the Issues area here to help others find it:
#38

FYI, I'm sure you both already know, but the fixes for V3 don't work with V4 for some reason. I tried to take a stab at it, but I can't figure out why the plugin won't register globally. I don't see any breaking changes that would cause it to cease working.

[MaximBelov]

Compatibility chart.js v3-v4

https://www.npmjs.com/package/hw-chartjs-plugin-colorschemes

#43