`Object.fromEntries` is showing up in Sentry as the most frequent error from
browsers [1] that are outside our supported range, so use it as a representative
feature to decide whether the client can load at all. We assume that this can
subsume the previous ES2015-era feature tests.

`Object.fromEntries` is available since Chrome 73 (Mar 2019), Safari
12.1 (Mar 2019), Firefox 63 (Oct 2018).

[1] https://sentry.io/organizations/hypothesis/issues/3747304654