Update shim/global.ts to use `globalThis`

[dasa]

Bug

Package Version: 5

Code

Chrome, Safari TP and TS are using globalThis rather than global. See: microsoft/TypeScript#29332 (This PR makes it sound like TS will handle this in the future such that the shim would no longer be needed.)

Expected behavior:

Actual behavior:

[dasa]

See: https://mathiasbynens.be/notes/globalthis

[maier49]

Resolved by #437