feat(kotlin): index Kotlin cross-file references

What does this MR do and why?

• Index Kotlin cross-file references
• Update Kotlin documentation with new relevant information to references
• Bumps gitlab-code-parser to 0.19.0

lated Issues

Resolves #156 (closed), gitlab-org&18074 (closed)

Testing

Locally and through unit tests

Performance Analysis

• This merge request does not introduce any performance regression. If a performance regression is expected, explain why.