feat: new body option for custom PR body, with placeholders

[bojanrajh]

This PR adds new body option for customizing pull request body while still supporting default text-blocks via placeholders.

      - name: Create Version Pull Request
        uses: changesets/action@v1
        with:
          body: '<!-- header --><!-- prestate --><!-- releasesHeading --><!-- body -->'

Fixes #65

[changeset-bot]

🦋 Changeset detected

Latest commit: b81dda4

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@changesets/action	Minor

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