[lit-analyzer] `npm run build:ts` fails after `lerna add`

[justinfagnani]

lit-analyzer installs multiple versions of typescript. Depending on which install tool/script is run, a different tsc binary will be written to node_modules/.bin/tsc. lerna add in any package seems to always cause the binary from typescript@~3.6.4 to be installed. This version doesn't support optional chaining, so the build fails. An npm run bootstrap from the project root will fix this.

We should consider putting the test versions of typescript into a separate subpackage to fix this.