feat(firebase_ui_auth): add `showPasswordVisibilityToggle` to `RegisterScreen`

[kzrnm]

Description

Currently, showPasswordVisibilityToggle is available on SignInScreen, but not on RegisterScreen.

Related Issues

114

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[russellwheatley]

@kzrnm - this seems a nice update. Could you merge with main branch, please?

[kzrnm]

@russellwheatley I have merged with main.