Add Time shortcode function to H2 toolbar

## Issue Description

Timezones are difficult. As we talk about unifying time formats across our comms, I can't for the life of me remember the shortcode for `[time]` when writing an H2. Is it possible to add time to the toolbar in H2 (see screenshot)
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Would be kind of cool to automatically parse content or create a custom markdown syntax so editors could enter time while writing.. who knows, I don't have the perfect UI in mind - just making it more accessible for users would be great.