feat(yaml): new hook onLanguageServiceCreated

[antfu]

Expose the language service instance created from volar-service-yaml

I needed it in slidevjs/slidev#1743 to it's API like registerCustomSchemaProvider

[remcohaszing]

I think this is what the custom provide is for, right? Having the ability to register custom schema providers makes sense. I haven’t actually gotten to using volar-service-yaml myself yet.

[antfu]

Yeah right, probably not needed at this moment.