Add `supportsMicrotasks` to the host config (#20809)

* Add `supportsMicrotasks` to the host config Only certain renderers support scheduling a microtask, so we need a renderer specific flag that we can toggle. That way it's off for some renderers and on for others. I copied the approach we use for the other optional parts of the host config, like persistent mode and test selectors. Why isn't the feature flag sufficient? The feature flag modules, confusingly, are not renderer-specific, at least when running the our tests against the source files. They are meant to correspond to a release channel, not a renderer, but we got confused at some point and haven't cleaned it up. For example, when we run `yarn test`, Jest loads the flags from the default `ReactFeatureFlags.js` module, even when we import the React Native renderer — but in the actual builds, we load a different feature flag module, `ReactFeatureFlags.native-oss.js.` There's no way in our current Jest load a different host config for each renderer, because they all just import the same module. We should solve this by creating separate Jest project for each renderer, so that the flags loaded when running against source are the same ones that we use in the compiled bundles. The feature flag (`enableDiscreteMicrotasks`) still exists — it's used to set the React DOM host config's `supportsMicrotasks` flag to `true`. (Same for React Noop) The important part is that turning on the feature flag does *not* affect the other renderers, like React Native. The host config will likely outlive the feature flag, too, since the feature flag only exists so we can gradually roll it out and measure the impact in production; once we do, we'll remove it. Whereas the host config flag may continue to be used to disable the discrete microtask behavior for RN, because RN will likely use a native (non-JavaScript) API to schedule its tasks. * Add `supportsMicrotask` to react-reconciler README