This fixes Element on older versions of Firefox by adding a Polyfill
for Intl.Segmenter. The Polyfill is conditionally imported so it only
inflates the initial bundle size by about 100 bytes. On browsers that
need it, the polyfill is quite large at 317Kb.
Users on these browser will still see the 'unsupported browser' screen,
but will be able to click through to use the app anyway. This keeps the
Intl.Segmenter modernizr check but this would also happen due to
https://github.com/element-hq/element-web/pull/27674
9f27685a54