fix(task-item): fix updating checked status

[nicksellen]

Changes Overview

The code currently uses setAttribute / removeAttribute to update the checked status of the checkbox, this is the incorrect method to use. Checkboxes have a checked property that can be set.

See https://aileenrae.co.uk/blog/programatically-check-uncheck-checkbox-javascript/

Implementation Approach

Switch to setting checkbox.checked property

Testing Done

Manually in browser.

Verification Steps

Open a collaborative session with task item extension available.

In one window check/uncheck the box a few times, then in the other window do the same.

Notice that the update appears to no longer propagate, the checkbox state does not change in the other window.

It is actually changing the attribute on the document node correctly, which you can see if have styling applied such as this (on the task item):

li[data-checked='true'] {
    text-decoration: line-through;
}

With the fix, it works again!

Additional Notes

Checklist

• I have created a changeset for this PR if necessary.
• My changes do not break the library.
• I have added tests where applicable.
• I have followed the project guidelines.
• I have fixed any lint issues.

Related Issues

I couldn't find an existing issue for it, which I thought was surprising, but maybe I didn't look well enough.

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