Remove use of globalThis

[Floriferous]

Related to #619.

The use of globalThis in the recent versions of uniforms-material has caused a bit of pain in our codebase including our testing setup.

I see that this is only used in one small place in DateField. From what I understand about it (never seen it before), it requires a polyfill to work outside of typescript (which you also use for your documentation website).

For this one single line in this repo, we have to import and use a polyfill, so is it really necessary?

[refactorized]

Had this issue too, and in my case just replaced globalThis with window. Not sure if that helps, but +1 on this issue from me.