Allow the fontweight for the title to be customized

[dylancom]

Description

Currently you can customize the fontFamily and fontSize but not the fontWeight (of the header title).
This PR let's you also change the fontWeight of the header title.

[WoLewicki]

Thanks for your contribution!

1. In the PR template, there are all the places where the change/addition of a property should be added. It looks like you've missed the READMEs of native-stack and createNativeStackNavigator.
2. Can you add this change to Android too? It should be rather simple. You can check here: https://github.com/software-mansion/react-native-screens/blob/master/android/src/main/java/com/swmansion/rnscreens/ScreenStackHeaderConfig.java#L236 and extend the if to check for both weight and FontFamily, only FontFamily (current), only weight.
3. If you would like, there is a TestsExample project on the newest master, if you rebase to it, you could add a Test706 file that shows a minimal application of the changes.

If you need any help, please comment here.

[dylancom]

@WoLewicki I tried Android but it had no effect, see: facebook/react-native#26193.
Seems that if you want for e.g. a bold font, you need to specify a bold font family.
I updated the READMEs, improved the iOS code and added an example.

[WoLewicki]

Looks good! Please apply the changes requested and this should be ready to merge then!

[dylancom]

@WoLewicki I applied the requested changes.

[WoLewicki]

Some minor tweaks.

[dylancom]

@WoLewicki Tweaked it.

[WoLewicki]

Ok it looks like it is ready to be merged 🎉 Thanks for your contribution!