Include meaningful User-Agent header in requests to the GitHub UI

This adds a custom `User-Agent` header to requests from the
GitHub server to the GitHub API, identifying the application,
the version and key information about the environment.

This aligns with the [recommendations][1] in the GitHub Docs.

As part of this change, I have also moved the current version of
the server into a constant, and fix the initialization of `Server`
to use that version, taking from `package.json`.

[1]: https://docs.github.com/en/rest/using-the-rest-api/getting-started-with-the-rest-api?apiVersion=2022-11-28#user-agent

src/github/common/version.ts

@@ -0,0 +1 @@
+export const VERSION = "0.6.2";