Fix LICENSE of published packages

[jakebailey]

The license on this repo is Apache 2.0, and we also prepend an Apache 2.0 copyright header onto these files when distributed with TypeScript. But, when the packages like @types/web were introduced, they were instead shipped with the MIT license, which isn't the right one for this code.

[DanielRosenwasser]

For all intents and purposes, I don't think it matters to most consumers. This was likely an oversight, but for consistency we should align with whatever lib.d.ts already ships as within TypeScript. Otherwise this will cause more issues down the line.