RemoteFlutterWidgets: Make Icon optional to enable tree shaking

### Use case

Currently, tree shaking completely breaks when using rfw's createCoreWidgets() function, which makes sense, because it includes dynamic IconData.

This is pretty bad when you're using a big icon library in your app, for example the highly popular [material_symbols_icons](https://pub.dev/packages/material_symbols_icons).
In our case, this would bloat the apk bundle size by 15.6 mb all by itself.

Switching to individual svg icons in a storage bucket somewhere is the much more sensible choice here in our case.

This seems to only be caused by `_coreWidgetsDefinitions` being an all or nothing approach instead of being modular.

Our quickfix was to copy the entire core_widgets.dart file into our repository and comment out the Icon widget. This worked, but now we need to keep this file up to date with upstream :(

## Related

- https://github.com/flutter/flutter/issues/162152
- https://github.com/flutter/flutter/issues/143677
(both closed due to inactivity)

I strongly believe that Icon shouldn't be in the list of core widgets by default, since it can unexpectedly bloat the app by a lot.

### Proposal

- Export each widget as a lowercase function, ie `animationDefaults(context, source)`.
Not sure what the benefit of `Map get _coreWidgetsDefinitions` is over such a list. Maybe slightly less code? Maybe it can be autogenerated slightly easier? I have no idea to be honest, but the downside is clear to me.
- `createSomeCoreWidgets` gets a list of these functions and it only constructs them
- There should be a `defaultCoreWidgets` list that doesn't contain the `Icon` widget

**Non breaking version:**
- `createCoreWidgets` by default still exports all

**Breaking version:**
- `createCoreWidgets` by default doesn't export the icon, but you can manually add it by passing icon() separately.