Add an API method to more easily invoke entrypoints

[shartte]

The functionality in EntryPointUtils is very useful to mods that define their own entrypoints too, since in most situations, they'll want to call them at some point.
This PR adds a public method to FabricLoader that provides the same functionality and removes the static utility class.

[liach]

Should we call it forAllEntrypoints? Since you can define a default method that returns a side-effect-free value. I see the main value as able to call all entrypoints even in cases of exceptions.

[Jab125]

should the EntrypointUtils class be deprecated instead of removed, to avoid mods that might reference fabric's internals?

[sfPlayer1]

No, the impl package is off limits overall. Loom could generate a wildcard discouraging (eclipse only?) or (opt-out) access restriction on fabric impl packages. We may also push more uses of the apistatus annotation.

[shartte]

No, the impl package is off limits overall. Loom could generate a wildcard discouraging (eclipse only?) or (opt-out) access restriction on fabric impl packages. We may also push more uses of the apistatus annotation.

One more reason to review and merge this 😈