build: update to typescript 4

[MarshallOfSound]

Lot's of fun stuff in typescript 4, this just updates us so we could in theory use it 😄

Notes: no-notes

[release-clerk]

No Release Notes

[vladimiry]

Re 8baa9de#diff-dd0e415ac6643adc75f041a4617e141bR168-R174 I think this would be a more type-safe way since you don't cast stuff to any (ie props get enumerated in a type safe way):

for (const prop of ["Buffer", "setImmediate", "clearImmediate", "global", "root", "GLOBAL"] as const) {
    delete (global as Partial<Pick<typeof global, typeof prop>>)[prop];
}