Add LSIF support to sourcegraph/typescript

[chrismwendt]

• ✅ sourcegraph/go has LSIF support as of Add LSIF support sourcegraph-go#85
• ❌ sourcegraph/typescript doesn't have LSIF support yet
• ✅ All other language extensions (including sourcegraph/python) have LSIF support by virtue of being generated from https://github.com/sourcegraph/sourcegraph-basic-code-intel

Spec:

• Fallback logic is in this order: 1. LSIF 2. language server 3. basic-code-intel (this follows other language extensions)
• Doesn't change UX for existing Sourcegraph instances that have no LSIF data even when codeIntel.lsif is enabled

@felixfbecker As the original author of sourcegraph/typescript, do you want to add LSIF support? If so, assign this to yourself, otherwise leave it assigned to me.

[felixfbecker]

I think it would be easier for you to add since you wrote the library and added basic-code-intel fallback too. Would be great to test the official Microsoft LSIF exporter for TypeScript