chore(doc-gen): put typescript stuff into its own package

This means that we can now run just the d.ts file generation by running:

```bash
gulp docs/typings
```

In addition the type definition generation was messing with the other docs tasks
so separating it also fixes problems there.

docs/docs-package/templates/lib/githubLinks.html

@@ -0,0 +1,3 @@
+{% macro githubViewLink(doc) -%}
+  <a href="https://github.com/{$ versionInfo.gitRepoInfo.owner $}/{$ versionInfo.gitRepoInfo.repo $}/tree/{$ versionInfo.currentVersion.isSnapshot and versionInfo.currentVersion.SHA or versionInfo.currentVersion.raw $}/modules/{$ doc.fileInfo.relativePath $}#L{$ doc.location.start.line+1 $}-L{$ doc.location.end.line+1 $}">{$ doc.fileInfo.relativePath $} (line {$ doc.location.start.line+1 $})</a>
+{%- endmacro -%}