Add callback for page changes in instructions plugin

[Shaobin-Jiang]

A new parameter page_change_callback is added to plugin instructions. This function is called every time a page change occurs and receives one single argument which is the index of the current page, starting from 0.

I am really in favor of this parameter because we often are in the need of acquiring the current page number and make corresponding changes. For example, we might want to hide button previous on the first page and change the label of button forward to "start experiment" on the last page. We can do this without the page_change_callback parameter naturally, but not as convenient as with the parameter.

Normally, with a plugin that does not have "page change", we can achieve this with the on_finish parameter. But given that the instructions plugin is a special one with page change, I think a resembling parameter ought to be added, hence the page_change_callback.

[changeset-bot]

🦋 Changeset detected

Latest commit: 1fd3b6d

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

This PR includes changesets to release 1 package

Name	Type
@jspsych/plugin-instructions	Minor

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[jodeleeuw]

Hey @Shaobin-Jiang this is great, thanks. What do you think about calling it on_page_change to make it consistent with other callback function parameters in the library?

[Shaobin-Jiang]

Great idea @jodeleeuw . Changed that bit in the latest commit.