VSCE: Add smoke test
Created by: philipp-spiess
This PR adds a smoke test for our VS Code extension. Note that this does not (yet) test product specific features but it asserts that:
- We can compile and build a working VS Code release bundle.
- The extension can be installed on the latest production VS Code version.
- We show the Sourcegraph icon in the left side menu bar and can click it.
- When clicked, we load the search home page and can assert that it contains the proper text content.
Test plan
Since this is adding an automated test, check out the code and the CI results. I expect the string "Test successful." to be logged when everything worked which can be seen in Buildkite: