Add editable preview mode to Markdown editor

[vishwamartur]

Related to #664

Add editable preview mode to the Markdown editor.

• New Command: Add editablePreview command in core/src/commands/preview.tsx to allow editing in preview mode.
• TextArea Component: Update core/src/components/TextArea/index.tsx to support rendering an editable preview mode.
• Editor State Management: Update core/src/Editor.tsx to manage the state and rendering of the editable preview mode.
• Context Management: Update core/src/Context.tsx to include support for editable preview mode in the context and state management.

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[itzcodex24]

@jaywcjlove Is this something we will see being merged in soon? Opened since Nov 2024. Not sure if this will resolve https://github.com/uiwjs/react-md-editor/issues/695