Show sidebar icon only when extension is active

[cbrnr]

Fixes #1572. Note that I am completely new to VSC extensions, so I've probably missed something, but I think this change is what other extensions like Julia or Python are doing.

I also tried to add "onLanguage:r" to the "activationEvents" array, but this does not seem to be necessary (it shows me a quick fix message "This activation event can be removed as VS Code generates these automatically from your package.json contribution declarations.").

[Fred-Wu]

This doesn't seem to be working when defined under the key "activitybar" as "when" is not an option value to it.

[cbrnr]

I copied that setting from the Julia extension: https://github.com/julia-vscode/julia-vscode/blob/main/package.json#L1315

I'm not saying that you are incorrect, but their activation mechanism is working for me. However, I do think that the r.isActive context is not defined anywhere. For reference, the Julia extension sets a similar context here.

Maybe it's sufficient to just activate it with

"activationEvents": [
    "onLanguage:r"
]