Bazel extension does not expose an API to other extensions

It's possible for extensions to expose direct execution from other VS Code extensions:
https://code.visualstudio.com/api/references/vscode-api#extensions

...but the Bazel extension does not offer any API calls (Extension.activate() does not return a value), so other extensions are forced to re-implement capabilities that are already present in the Bazel extension, such as:
- Locating the WORKSPACE file
- Watching for BUILD file changes
- Querying for targets in a BUILD file
- etc...

Start by reading the VS Code extensions API documentation linked in the issue and inspect the extension's Extension.activate() entry point. Determine which capabilities should be exposed for other extensions, how callers would access them, and what completion criteria and tests are needed for the API design.

Written by the indexing model from the issue text.