Updating default features to include WASM_BIGINT, bulk-memory, and nontrapping-fptoint

[dschuff]

We will shortly update emscripten to enable these features by default, raising the default browser versions (i.e. settings.MIN_CHROME_VERSION etc) to Safari 15.0, Chrome 85, Firefox 79, and Node 16. (The only default that will
actually change is Safari, from 14.1 to 15).

In order to try to minimize the number of things happening simultaneously, I plan to land each of #22993, #22873, and #23007 separately. Before all of them are landed, I plan to not adjust the defaults for MIN_SAFARI_VERSION but instead reduce the version required for each feature separately (e.g. the bulk memory PR will reduce the Safari version for bulk memory from 15 to 14.1 rather than raising the system-wide default from 14.1 to 15). This will have the effect that each feature will still be controlled by the feature_matrix code but will be enabled separately. Then once all 3 are enabled together and stable, I'll raise the default and fix the individual features so that manually setting MIN_SAFARI_VERSION will have the correct effect. I plan not to make any releases while we are in this state.