Make src-cli installable via npm.

Created by: olafurpg

Fixes #759 (closed). Previously, the recommended way to install src-cli on Linux was to download a static binary via curl. This PR makes it possible to install src-cli via npm install -g @sourcegraph/src. The new npm distribution will be particuarly helpful for users of our new scip-typescript and scip-python indexers that are also distributed via npm.

Test plan

Manually ran yarn install in the npm-distribution directory to verify that it works as expected. There's no reliable way to test this change without trying to run the actual release step to npmjs.com.