Add Slash commands for /time and /date shortcuts

[paulm]

Add the same Slash menu functionality as Day One. See: https://github.com/bloom/DayOne-Apple/issues/22096

• Add on iOS
• Add on macOS
• Add on Android
• Add on Web

[peril-automattic]

	Fails
🚫	Please add a feature label to this issue. e.g. 'Stats'

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