Create a System Builder Syntax instead of using System Descriptors

[thechubbypanda]

Objective

Closes #1978

Solution

Working around @cart's [design](https://github.com/bevyengine/bevy/issues/1978#issuecomment-825337056], implement a builder syntax for systems such that we can do something like:

App::new().add_system(
    foo.add_to_stage(Stage::Label)
        .before(Stage::Label2)
        .startup()
)

Please do watch this pr and comment on the changes made :)
Made with love and significant help from @Ratysz XD

[thechubbypanda]

@alice-i-cecile, could you please clarify what you mean by

main docs for each configuration method

and also

point to the configuration methods for each relevant concept

from where?

[alice-i-cecile]

main docs for each configuration method

So, this is "the method to add a system to a stage should point to the Stage docs" and so on.

point to the configuration methods for each relevant concept

from where?

From e.g. the Stage docs. The idea here is that we want bidirectional links between "the thing we're configuring" and "the tool used to configure it".

[thechubbypanda]

Documentation checklist

I believe all are now complete bar the book :)

[IceSentry]

Could this also be ported to SystemSet? That would allow things like this:
Nevermind, it's already done.

I'm also curious what happens if a system in a SystemSet specifies a stage like so

.add_system_set(
	SystemSet::new()
		.stage(Stage::Label)
		.with_system(example_system.stage(Stage::OtherLabel)))

[Ratysz]

.add_system_set_to_stage() shouldn't be a thing anymore. If it is, that's an oversight in the PR.

[IceSentry]

@Ratysz yeah, sorry that was copy pasted from the first part of my question before I noticed it was removed and I removed that part of my comment. It's not in the code anymore.

[Ratysz]

Right. I don't remember if it came up during implementation, but that should crash, the same as with extra run criteria.

[thechubbypanda]

We don't actually throw any errors when a system has it's own stage and is also inside a set. On baking the SystemSet, we either overwrite or add to data in the systems.

[Ratysz]

Well... I think both the system and the set it's in having a stage label should be a hard error, because it's code that doesn't have an obvious correct pathway. See here for similar discussion about run criteria.

[IceSentry]

It's not particularly important to fix this, but some of the impl block have inconsistent spacing between functions. MOst of the codebase has a space between each function.

Overall, I really like this new api.

[IceSentry]

I think this could keep a reference to the conflicting stage name and add that to the panic message.

Small suggestion, you could use this instead conflict = set.systems.iter().any(|system| system.config().stage != set.config().stage);, but that's entirely up to preference.

[alice-i-cecile]

Fixed by #7267.