Improve dev experience for picking up new Kallichore versions

It was noted that neither `npm install` nor even `./rebuild.sh` was sufficient to pick up a new Kallichore version for one developer; only this did it: ```bash VSCODE_FORCE_INSTALL=1 npm install ``` This is due largely to the new install caching added recently upstream. Now running `npm install` doesn't necessarily install everything; it just checks to see if anything _needs_ installing. Since Kallichore is installed as a side effect (a post install script), a version mismatch will not necessarily trigger a reinstall. To address this, we should: - amend rebuild.sh so that it deletes built prebuilt copies of kallichore, ark, etc. (preserving its purity as a "clean install") - include the kallichore version in the extension install hash computation so that changing it triggers a reinstall (and therefore a rerun of the post install script)