Code: consider activating the extension lazily

[lnicola]

We currently set workspaceContains:**/Cargo.toml to have the extension activate if Rust code is present in the workspace, but Code will time out after 5 seconds and activate it anyway.

We could try starting only when editing a Rust file, like vscode-rust did in rust-lang/vscode-rust@61838af.

Thanks to @kankri for the awesome investigation in #2533 (comment), which I've unfortunately missed and to @ishowta who dug it up.

Related #2533
Related #7268

[taikulawo]

Any updates in there? It's annoying to showing rust-analyzer failed to discover workspace in no-rust project

[matklad]

I wonder if we can ask vscode to look for cargo toml only one level deep? I am not a huge fan of loading only when a file is open: I often start with “workspace symbol” request to figure out which file I want opened in the first place.