Add integration tests with the CLI

This commit adds integration tests that run commands using the CLI. This
change introduces a number of enhancements in order to get there.

1. Augments the index-template.ts file so that it downloads an
appropriate cli version if requested.
2. Adds the ensureCli.ts that performs the download if a a suitable
version is not already installed. See the comments in the file for how
this is done.
3. Changes how run-integration-tests is done so that the directories
run are specified through a cli argument.
4. Updates the main.yml workflow so that it also runs the
cli-integration tests.
5. Takes advantage of the return value of the call to `activate` on the
extension. This allows the integration tests to have access to internal
variables of the extension like the context, cli, and query server.
6. And of course, adds a handful of simple tests that ensure we have a
cli installed of the correct version.

.gitignore

@@ -4,6 +4,7 @@
 # Generated files
 /dist/
 out/
+build/
 server/
 node_modules/
 gen/