feat: adapter features, deprecate astro configs

[ematipico]

Changes

This PR deprecates build.split and build.excludeMiddleware.

After an internal discussion, we realised that these two config values should not live inside Astro, because they are only usable only with an adapter that can understand it

Because of that, we introduced the concept of adapterFeatures. These features are toggles that signal Astro to change the output of the build, and then the adapter needs to do something with those.

Testing

I updated the current test to use the new configuration. The tests should pass.

Docs

/cc @withastro/maintainers-docs for feedback!

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[bluwy]

Thanks for making the changes! This looks great to me.

Should we also validate if the deprecated configs are used in runtime, somewhere around here?

astro/packages/astro/src/core/config/config.ts

Lines 65 to 77 in 73eb4df

	let legacyConfigKey: string | undefined;
	for (const key of Object.keys(userConfig)) {
	if (LEGACY_ASTRO_CONFIG_KEYS.has(key)) {
	legacyConfigKey = key;
	break;
	}
	}
	if (legacyConfigKey) {
	throw new AstroError({
	...AstroErrorData.ConfigLegacyKey,
	message: AstroErrorData.ConfigLegacyKey.message(legacyConfigKey),
	});
	}

Also, the docs team were also interested in the API discussion for this, so might be good to run by them if this new syntax is better. cc @withastro/maintainers-docs

[ematipico]

@bluwy

Should we also validate if the deprecated configs are used in runtime, somewhere around here?

Two reasons why we can't do that:

• these keys throw an error, which means it won't be a deprecation;
• the code reads only the first level of keys, which doesn't fit our case because both keys are inside build

[TheOtterlord]

Not core docs, but I found a few typos

[TheOtterlord]

Suggested change

	* Creates and edge function that will communiate with the Astro middleware
	* Creates an edge function that will communicate with the Astro middleware

[TheOtterlord]

typo?

Suggested change

	// TODO: remover in Astro 4.0
	// TODO: remove in Astro 4.0

[bluwy]

Two reasons why we can't do that:

• these keys throw an error, which means it won't be a deprecation;
• the code reads only the first level of keys, which doesn't fit our case because both keys are inside build

I mean we can to the warning deprecations, and validate build.* around that area of the code, not necessarily error-ing like it. But it would be a nice to have for me and not something needed.