Run typecheck on CI and fix issues

[silverwind]

1. type checking (deno check) was not running on CI and there were 7 type errors which I've fixed and now it's running
2. GitHub API types are sourced from https://github.com/octokit/types.ts and I created basic types for Issue and PullRequest.
3. IDK about those two webhook errors, so I used @ts-expect-error.
4. deno.lock was auto generated, I guess it's good to have it in-repo.
5. actions/checkout is upgraded to latest major.

[silverwind]

Weird failure on actions:

Run deno check src/**/*.ts
  deno check src/**/*.ts
  shell: /usr/bin/bash -e {0}
error: Module not found "file:///home/runner/work/gitea-backporter/gitea-backporter/src/**/*.ts".

[silverwind]

Fixed. The bash where this runs on needed shopt -s globstar to enable recursive globs. deno check itself is too dumb to typecheck a directory (see https://github.com/denoland/deno/discussions/12113).