refactor(language-core): make embeddedCodes optional in VirtualCode

[remcohaszing]

In many cases embeddedCodes is always empty. It might as well be optional.

[rchl]

Breaking change (maybe someone relies on it being an empty array and uses length check)? If so, is it really worth changing?

[remcohaszing]

For Volar VirtualCode is input. volar-services doesn’t use embeddedCodes anywhere. So this could indeed be a breaking change, but only on a type level. This is easy to resolve using a non-null assertion or optional chaining.

[johnsoncodehk]

I agree that embeddedCodes should be optional, the purpose of it being required in the past was to guide developers to implement embedded code (due to lack of documentation). But I'm sure we'll make progress on the documentation soon.

Since this is a breaking change, this will be released in 2.1 (Volar does not follow semantic versioning)