BSN: changing prefixes for Template and SceneComponent to allow removing confusing "inheritance" concept

[laundmo]

Objective

Changes discussed with cart:

• : will not be called "inheritance" anymore, but instead "cacheable" (name not final)
• the Template patching prefix changes @ -> ~
• the SceneComponent prefix changes : -> @
• the : prefix is allowed in front of @SceneComponent as well, marking it as being cached

me: do we forbid caching of functions which take an argument?

cart: We should forbid it until we support it, and I don't plan on supporting this in the short term, as it adds new controversial design aspects to the system (building a mapping from parameters to cached scenes, ensuring the parameters are all hashable, etc).

Link to the discord thread, specifically my recap message.

The exact reasoning is in the linked discord discussion, but the tldr is:
"Inheritance" as a term for the ":" syntax didn't really fit what bsn was doing, since scenes without : produce the same result. All : is, is a way to say "cache this pls" with the design of bsn imposing the limitation that only the first scene can be cached

That was all fine, except for one case: Disambiguating SceneComponents from normal components. So if we redefine : to just mean "caching" we needed to untangle it from SceneComponents. That means disambiguating SceneComponents needed a different syntax, and the decision ended up being to take the @ prefix from the much less common @Template usecase, since SceneComponent {@prop: val} already used @ as well and introduce a new prefix to replace @Template instead, replacing it with ~Template.

Solution

Implement these changes in macros.

Notably, this still doesn't implement/enable caching right now.

Most docs are updated, except for the big blocks in bevy_scene/src/lib.rs and bevy_scene/macros/src/lib.rs since i've been working on reworking those on another branch and didn't want to duplicate whats already been a lot of work and likely to be a bunch more

Testing

• bevy_scene tests passing
• cargo check --workspace and cargo check --workspace --examples passing

Since this PR does not yet enable caching, I think passing the current tests should be enough.

[laundmo]

CI is expected to fail for a while, until all tests, uses, etc are migrated.

Will likely impact currently open PRs which use BSN, as it changes syntax, unless they don't use : with arguments or SceneComponents.

[cart]

This is a solid step forward! Just pushed a minor tweak.

[Zeophlite]

From Discord :

I want to save -Thing for "removal patches"

~Thing
-Thing

These are visually very similar, it might be worth considering a different symbol as followup

[Zeophlite]

i think this greatly improves developer intuition on the concept