publish @sourcegraph/extension-api-types package with plain types

Created by: sqs

This is like the vscode-languageserver-types package. It exports types that are useful for client applications interacting with Sourcegraph extensions. Sourcegraph extensions DO NOT and SHOULD NOT use this package. Only things like codeintellify (and our internal code) should use it.

The problem it solves is that these types were duplicated in many places in our internal code and libraries. They will now be canonically defined in this package. (This is the same reason why vscode-languageserver-types was created by the VS Code team.)