refs TryGhost/Team#2904
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### <samp>🤖 Generated by Copilot at 2ba5e2a</samp>
This pull request adds the lexical editor feature to the Ghost admin
app, which allows users to create and edit snippets in a natural
language format. It modifies the `snippet` model, adapter, and
controller, and the `lexical-editor` template and component to support
the new feature.
