This adds a simple calculator widget, somewhat similar to the one presented
when searching calculator on Google.
Also, it adds somewhat of a template for making the addition of new widgets
easier via the app/utils/widgets.py file. My eventual plan is to use this to
create more widgets that appear in Google, such as a color picker, timer, etc.
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Co-authored-by: Ben Busby <contact@benbusby.com>
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