Please support File References

#### Reason/Context
Please try answering few of those questions
- Why we need this improvement?
- AsyncAPI files can get very long very quickly -- which makes readability difficult
- It would be great to be able to extract out a collection of common `schemas` and then use it in multiple files

#### Description
Please try answering few of those questions
- What changes have to be introduced?
- Be able to load multiple files -- preferably an entire subdirectory tree -- into the editor
- Will this be a breaking change?
- Unknown
- How could it be implemented/designed?
- Unknown