Add is_workspace_member for rust-project.json
Add is_workspace_member for rust-project.json

It is currently unused, but, in the future, it will be used to:

* drive certain UX (symbols search by default will look only in the
members)
* improve performance (rust-analyzer will assume that non-members
change rarely)

If not specified, is_workspace member is inferred from the path

rust-analyzer/rust-analyzer,rust-analyzer/rust-analyzer,rust-analyzer/rust-analyzer,rust-analyzer/rust-analyzer,rust-analyzer/rust-analyzer