Move convertGithubNwoToDatabaseUrl to databases/github-databases/api.ts

[robertbrignull]

(Builds upon the https://github.com/github/vscode-codeql/pull/3452 PR because otherwise it'll just conflict. The changes aren't related except that the functions being moved are next to each other.)

I argue that the convertGithubNwoToDatabaseUrl function (and promptForLanguage which it uses) aren't particularly about fetching or importing databases and are more about interacting with the GitHub API, though I do realise the distinction isn't particularly clear.

These functions also stand out from the rest of database-fetcher.ts because they don't require arguments like DatabaseManager, storagePath: string, or CodeQLCliServer, while almost all of the other functions do.

I think it's neater to pull these functions out and move them to databases/github-databases/api.ts instead. This puts them next to listDatabases which makes sense as they largely just act on the output of that function.

Checklist

• [ ] CHANGELOG.md has been updated to incorporate all user visible changes made by this pull request.
• [ ] Issues have been created for any UI or other user-facing changes made by this pull request.
• [ ] [Maintainers only] If this pull request makes user-facing changes that require documentation changes, open a corresponding docs pull request in the github/codeql repo and add the ready-for-doc-review label there.