Remove `AssetServer::watch_for_changes()`

[irate-devil]

Objective

AssetServer::watch_for_changes() is racy and redundant with AssetServerSettings. Closes #5964.

Changelog

• Remove AssetServer::watch_for_changes()
• Add AssetServerSettings to the prelude.
• Minor cleanup.

Migration Guide

AssetServer::watch_for_changes() was removed. Instead, use the AssetServerSettings resource.

app // AssetServerSettings must be inserted before adding the AssetPlugin or DefaultPlugins.
	.insert_resource(AssetServerSettings {
		watch_for_changes: true,
		..default()
	})

[irate-devil]

could we have a method so that enabling hot reloading doesn't require 4 lines of boilerplate?

It's tempting to add a method like that, but then it becomes awkward when trying to set other settings, requiring a refactor.

Maybe consider deprecating?

Francois argues that we should just break things. It makes sense to me to avoid the maintenance cost of deprecation :)

[alice-i-cecile]

bors r+

[bors[bot]]

Build failed (retrying...):

• build-and-install-on-iOS

[bors[bot]]

Build failed (retrying...):

• build (macos-latest)

[bors[bot]]

Pull request successfully merged into main.

Build succeeded:

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