Support `exempt-issue-types` option

[veryfancy]

Description:

#1187 added support for a only-issue-types option.

Other similar options like only-issue-labels have blocklist counterparts, like exempt-issue-labels. We can do the same and give only-issue-types a counterpart exempt-issue-types.

Justification:

My team wanted to use this to prevent certain types of issues from being closed as stale. However, we have a lot of untyped issues, and configuring our workflow using an only-issue-labels allowlist instead of a blocklist looks like it will prevent untyped issues from being closed as stale. That's not the behavior we'd prefer.

Are you willing to submit a PR?

Yes, I have provided a PR.

[aparnajyothi-y]

Hello @veryfancy, Thank you for creating this issue and we will get back to you with some feedback :)

[aamotharald]

My team has the same use case :-)