Add support for merge_group event

[espenalb]

The get-diff-action currently ignores events from merge_group - this is unfortunate when using the (excellent) GitHub merge queue concept, as our checks are skipped because the git diff is reported as empty.

Fixing this (I think) is as simple as adding merge_group to TARGET_EVENTS in src/constant.ts

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[espenalb]

I have published my forked version to the Github marketplace, https://github.com/marketplace/actions/get-diff-action-including-merge_group-merge-queue because I was concerned that this PR would not be merged. (Since there does not seem to have been any activity here for the last 12months).