refactor(workspace-tree): Refine event and dispose handling

[cwahbong]

Less dependency for BazelWorkspaceTreeProvider, making the future testing changes simpler.

Also small clean ups for this capturing and making the styles (naming and initializations) like
https://github.com/microsoft/vscode-extension-samples/blob/2f83557/tree-view-sample/src/nodeDependencies.ts

[cwahbong]

The rest of work for workspace handling external dependency is still without test cases (it-works-on-my-machines), thus uploading only a change without much behavior change first.

[vogelsgesang]

The rest of work for workspace handling external dependency is still without test cases (it-works-on-my-machines), thus uploading only a change without much behavior change first.

Thanks for the context! Sounds good. Even independent of any future changes, this is definitely an improvement. Great work, keep it going 🙂

By the way: You can reach me and the other maintainers of this extension on https://slack.bazel.build/, in case you want to discuss architecture / code design / ...