Merge definitions of Disposable where possible

[robertbrignull]

We had three identical versions of Disposable types, all with the intention of avoiding a dependency on VS Code. I propose we can merge them all and just use the one from disposable-objects.ts. That file is already in common and has a comment at the top saying to avoid dependencies on VS Code.

Checklist

• CHANGELOG.md has been updated to incorporate all user visible changes made by this pull request.
• Issues have been created for any UI or other user-facing changes made by this pull request.
• [Maintainers only] If this pull request makes user-facing changes that require documentation changes, open a corresponding docs pull request in the github/codeql repo and add the ready-for-doc-review label there.

[charisk]

I think the intention with packages/commands is that it's a stand-alone package that we could potentially publish to npm one day. So I think we need to leave this implementation here.

[robertbrignull]

Ok, I'll revert that one.

It would be better for the future if this requirement were enforced or documented in the code, so we don't have to rely on remembering it.

[charisk]

Yep, agreed!

Revoking my approval since @charisk brings up a point I hadn't considered.

[robertbrignull]

PR is updated to just remove the instance from log-scanner.ts

[charisk]

Thank you for tidying that up!