symmetry span task is broken

[Sam Zorowitz]

After the update to jsPsych v7+, the symmetry span task has not worked.

[Sam Zorowitz]

The symmetry span task has been moved to a separate branch (symmetry-span). Anyone willing to fix the code is welcome to do so! In the meantime, the code has been removed from the main branch (#92).

This also takes care of the following two Dependabot alerts for now:

• https://github.com/nivlab/jspsych-demos/security/dependabot/2
• https://github.com/nivlab/jspsych-demos/security/dependabot/3