Change ProgressIndicator in DynamicIsland from struct to final class

[helloworldjay]

Hi!
Thx for your awesome codes for Dynamic Island :]

I just figured it out that it's better to change ProgressIndicator which is implemented in extension DynamicIsland.
Since the property progressIndicator is declared as let property which is struct, it's impossible to allocate new values for properties in progressIndicator as README.md you wrote.

We can make progressIndicator var as well, but I thought this way is better(But it's your call of course!).

I hope it's not my misunderstanding and can help u even though it's just a little thingy ☺️

Plz let me know if there are any changes you need more or this PR is not needed.
Thx again!

README.md

A simple object that provides access to a progress indicator around the Dynamic Island cutout.
To use it, simply access DynamicIsland.progressIndicator from anywhere to control the indicator.

DynamicIsland.progressIndicator.progressColor = .green
DynamicIsland.progressIndicator.isProgressIndeterminate = false