Don't match every Jira issue found in PR description

_(continuing from https://github.com/atlassian/github-for-jira/issues/119#issuecomment-1010360037)_

Recently you released a feature where a Jira issue ID like `TEST-12` can be included in the description of a PR and automatically linked to the issue.

This is great, but it's common to refer to multiple tickets within the description of a PR, and the PR itself only be associated with one Jira ticket.

Personally, I would only expect a PR to be linked to a Jira issue if it matched something like `fixes [TEST-12]` or `closes [TEST-12]`. A special keyword would ideally be required before the issue ID, and the issue ID be "linked" i.e. wrapped with `[` and `]`.

Any chances you could refine this feature to provide more control over this linking?

Start by reviewing the existing Jira-linking behavior for pull request descriptions and the continuation discussion linked in the issue. Define the accepted keyword and bracket syntax, then verify that references without the required form are not linked while matching references are linked to the pull request.

Written by the indexing model from the issue text.