core(config): only allow lighthouse:default extension

[patrickhulce]

Summary
Closes the loop on a few consequences @paulirish and I discussed of #11779. Breaking change to eventually allow extension of lighthouse:mobile and lighthouse:desktop but make it clearer when your extends property is valid.

Before
Any truthy value for a config's extends property would trigger extension of the default config.

After
Any value other than lighthouse:default for a config's extends property throws a fatal error.

Related Issues/PRs
ref #11779
#10910 (comment)

[brendankenny]

Oh good call on getting this in before 7. Drop the boolean from

lighthouse/types/config.d.ts

Line 20 in ced75b5

extends?: 'lighthouse:default' | string | boolean;

too?

[adamraine]

Can you also update:

lighthouse/docs/configuration.md

Lines 49 to 53 in 6e0158d

	### `extends: "lighthouse:default"|boolean|undefined`
	The `extends` property controls if your configuration should inherit from the default Lighthouse configuration. [Learn more.](#config-extension)
	Both the values `"lighthouse:default"` and `true` will enable inheritance, while `false` and `undefined` will not.

[patrickhulce]

Done and done, thanks @brendankenny @adamraine ! 👍

[adamraine]

LGTM