Document dependencies search BETA feature

Created by: tsenart

1. https://docs.sourcegraph.com/code_search/reference/language#built-in-repo-predicate
2. https://docs.sourcegraph.com/code_search/reference/queries
3. Mention lazy syncing of package repos via dependency searching in doc/integration/npm.md (cc @olafurpg)
4. CHANGELOG entry
5. Top level feature in https://docs.sourcegraph.com/code_search
6. Create new https://docs.sourcegraph.com/code_search/how-to/dependencies-search
   1. How to set up
      1. Create NPM code host connection
   2. Usage
      1. Screenshots / Video / Gif / Loom
   3. Current limitations
      1. Works with Javascript and Typescript projects that have yarn.lock and package-lock.json files only.
   4. Roadmap for this feature
      1. Language support
      2. Predicate DSL / extra features
         1. scope:
         2. file:
7. Add all how-tos to How-tos section in https://docs.sourcegraph.com/code_search