feat(language-service): add provide generic to ServicePlugin type

[remcohaszing]

This allows a service plugin instance provide value to be inferred without the need for an explicit type annotation on the create method.

[johnsoncodehk]

Thanks! btw I recommend always type-annotating the create method, otherwise unknown options cannot be caught:

const servicePlugin: ServicePlugin = {
	create() {
		return {
			aTypoOption: 123, // Expected report type error but didn't
		};
	},
};

const servicePlugin: ServicePlugin = {
	create(): ServicePluginInstance {
		return {
			aTypoOption: 123, // Now report as expected
		};
	},
};