Add typed shortcut expansions

[astroboii47]

Summary

• add user-managed text snippets with configurable shortcut trigger
• show snippet suggestions while typing shortcuts and allow selecting/replacing them from the keyboard
• add emoji and symbol shortcode lookup assets
• show an inline shortcode popup while typing colon-prefixed shortcuts
• expand completed shortcuts like 😄 and allow Space to accept the top shortcode suggestion
• include typed shortcut settings/data in backup/restore data

Notes

• this combines snippets and emoji/symbol shortcodes as one typed-shortcuts feature area
• this PR is independent from the GIF/media picker and prediction PRs

Testing

• ./gradlew :app:assembleDebug

[astroboii47]

Closing in favor of #259, which contains the cleaned combined feature set with split commits.