Consolidate code intel extensions

[efritz]

In #198, the interface to activate a code intelligence extension was simplified so that sourcegraph-go and sourcegraph-typescript extensions can use the same LSIF/search fallback logic without duplication. This helps have a central place for logic where changes such as UI badges and telemetry events can happen with lower developer effort and a reduced chance of mistakes.

This action was preluded by this slack thread and by a declaration in this comment on that PR.

This is part one of the consolidation effort. This PR specifically performed the following actions:

• Copied the go extension code from the branch of Activation refactor sourcegraph-go#117 to extensions/go
• Copied the typescript extension code from the branch of Activation refactor sourcegraph-typescript#308 extensions/typescript
• Moved package/ to shared/
• Consolidated all dependencies in the root-level package.json and symlinked the node_modules/.bin in each template to the same directory in the root (a la sg/sg)
• Made the top-level tsconfig a composite project (and updated each extension tsconfig to reflect this)
• Removed the necessity for goreman to run extensions. Now run yarn run serve in the extension directory.
• Rewrote the generator and publish scripts, now in scripts/ and invocable via yarn commands
• Moved languages.ts into a package
• Updated the readme developer instructions
• Updated buildkite pipeline (WIP)
• Update prettier, tslint, eslint, mocha configs

Next steps will include:

• Removing the extension code from sourcegraph-go and sourcegraph-typescript, or marking them as deprecated entirely
• Finding a common interface to use the work started by lsp-client. This will either require using it as a dependency, moving the code in tree and archiving the source repo. I do not see an explicit path forward that does not modify lsp-client in some way (to either proxy the registration methods or to add the fallback logic directly to the LSP feature definitions). This requires additional review.
  • see Use lsp-client sourcegraph-go#36
  • see Use @sourcegraph/lsp-client sourcegraph-typescript#106

[chrismwendt]

Having everything in one repo is going to make development much nicer ✨

• I updated the README a bit
• I verified yarn run serve works