buffer: use private properties for brand checks in File
The current checks wouldn't work for certain values, and other places in the code have moved to this behavior as well.
import { File } from 'node:buffer'
const pd = Object.getOwnPropertyDescriptor(File.prototype, 'name')
pd.get.call(true) // Uncaught TypeError: Cannot use 'in' operator to search for '#name' in true
pd.get.call(null) // Uncaught TypeError [ERR_INVALID_THIS]: Value of "this" must be of type File