Url for PRs are wrong

[AlexanderBrevig]

Currently the LSP will generate an /issues/ URL when completing a PR.

This is because we're using the issues API.
https://github.com/AlexanderBrevig/github-lsp/blob/e5b99c94f233c93fec3aed1936e81218fc816c57/src/backend.rs#L260-L267

Maybe we should use search and filter on is:issue and is:pr, then decorate the cached object in
https://github.com/AlexanderBrevig/github-lsp/blob/e5b99c94f233c93fec3aed1936e81218fc816c57/src/backend.rs#L20

[AlexanderBrevig]

Not marking this as bug, since it is working. GitHub rewrites the URL and points to the correct PR.