[Feature Request][move-analyzer] Support in-memory files in virtual file system

[awelc]

# 🚀 Feature Request

At this point, virtual file system (VFS) used by the Move analyzer abstracts on-disk files only but it really should also support in-memory files.

Motivation

The main motivation here is that at this point metadata needed for go-to-def, find-references, etc. is re-computed on file save. In more mature code editing environments (e.g., in rust-analyzer), this information is updated even if no file save happens - instead it happens on key-stroke (typically not on each key-stroke, though).