Support "issue_comment" event

If an action is triggered via `issue_comment`, the PR number object might be different.

**CI File**

```
on:
issue_comment:
types:
- created

jobs:
build-comment:
if: |
github.event.issue.pull_request &&
(github.event.comment.body == 'ping')
runs-on: ubuntu-latest
permissions:
contents: write
id-token: write
pull-requests: write

steps:
- uses: actions/checkout@v2

- name: Comment to PR with zipped file
uses: exercism/pr-commenter-action@v1.3.0
with:
github-token: ${{ github.token }}
config-file: ".github/pr-commenter.yml"
```

**Technical Details**

The `getPRNumber` function needs to be updated https://github.com/exercism/pr-commenter-action/blob/47beb19785cef0671280078fe6dfa0791052b023/lib/run.js#L78

```ts
function getPrNumber() {
const pullRequest = github.context.payload.pull_request || github.event.issue;
if (!pullRequest) {
return undefined;
}

return pullRequest.number;
}
```

**GitHub Context Dump**

```json
{
"event": {
"action": "created",
"comment": {
"author_association": "CONTRIBUTOR",
"body": "e2e",
"created_at": "2022-04-26T08:47:20Z",
"html_url": "https://github.com/org/repo-name/pull/173#issuecomment-1109524822",
"id": 0,
"issue_url": "https://api.github.com/repos/org/repo-name/issues/173",
"node_id": "",
"performed_via_github_app": null,
"reactions": {
"+1": 0,
"-1": 0,
"confused": 0,
"eyes": 0,
"heart": 0,
"hooray": 0,
"laugh": 0,
"rocket": 0,
"total_count": 0,
"url": "https://api.github.com/repos/org/repo-name/issues/comments/1109524822/reactions"
},
"updated_at": "2022-04-26T08:47:20Z",
"url": "https://api.github.com/repos/org/repo-name/issues/comments/1109524822",
"user": {
"avatar_url": "https://avatars.githubusercontent.com/u/13672022?v=4",
"events_url": "https://api.github.com/users/user/events{/privacy}",
"followers_url": "https://api.github.com/users/user/followers",
"following_url": "https://api.github.com/users/user/following{/other_user}",
"gists_url": "https://api.github.com/users/user/gists{/gist_id}",
"gravatar_id": "",
"html_url": "https://github.com/user",
"id": 13672022,
"login": "user",
"node_id": "",
"organizations_url": "https://api.github.com/users/user/orgs",
"received_events_url": "https://api.github.com/users/user/received_events",
"repos_url": "https://api.github.com/users/user/repos",
"site_admin": false,
"starred_url": "https://api.github.com/users/user/starred{/owner}{/repo}",
"subscriptions_url": "https://api.github.com/users/user/subscriptions",
"type": "User",
"url": "https://api.github.com/users/user"
}
},
"issue": {
"active_lock_reason": null,
"assignee": null,
"assignees": [],
"author_association": "CONTRIBUTOR",
"body": "",
"closed_at": null,
"comments": 3,
"comments_url": "https://api.github.com/repos/org/repo-name/issues/173/comments",
"created_at": "2022-04-26T08:31:23Z",
"draft": false,
"events_url": "https://api.github.com/repos/org/repo-name/issues/173/events",
"html_url": "https://github.com/org/repo-name/pull/173",
"id": 0,
"labels": [],
"labels_url": "https://api.github.com/repos/org/repo-name/issues/173/labels{/name}",
"locked": false,
"milestone": null,
"node_id": "",
"number": 173,
"performed_via_github_app": null,
"pull_request": {
"diff_url": "https://github.com/org/repo-name/pull/173.diff",
"html_url": "https://github.com/org/repo-name/pull/173",
"merged_at": null,
"patch_url": "https://github.com/org/repo-name/pull/173.patch",
"url": "https://api.github.com/repos/org/repo-name/pulls/173"
},
"reactions": {
"+1": 0,
"-1": 0,
"confused": 0,
"eyes": 0,
"heart": 0,
"hooray": 0,
"laugh": 0,
"rocket": 0,
"total_count": 0,
"url": "https://api.github.com/repos/org/repo-name/issues/173/reactions"
},
"repository_url": "https://api.github.com/repos/org/repo-name",
"state": "open",
"timeline_url": "https://api.github.com/repos/org/repo-name/issues/173/timeline",
"title": "ci: dummy test",
"updated_at": "2022-04-26T08:47:20Z",
"url": "https://api.github.com/repos/org/repo-name/issues/173",
"user": {
"avatar_url": "https://avatars.githubusercontent.com/u/13672022?v=4",
"events_url": "https://api.github.com/users/user/events{/privacy}",
"followers_url": "https://api.github.com/users/user/followers",
"following_url": "https://api.github.com/users/user/following{/other_user}",
"gists_url": "https://api.github.com/users/user/gists{/gist_id}",
"gravatar_id": "",
"html_url": "https://github.com/user",
"id": 0,
"login": "user",
"node_id": "",
"organizations_url": "https://api.github.com/users/user/orgs",
"received_events_url": "https://api.github.com/users/user/received_events",
"repos_url": "https://api.github.com/users/user/repos",
"site_admin": false,
"starred_url": "https://api.github.com/users/user/starred{/owner}{/repo}",
"subscriptions_url": "https://api.github.com/users/user/subscriptions",
"type": "User",
"url": "https://api.github.com/users/user"
}
},
}
```