Add support for turning back to Markdown

The CommonParser class has a toHtml method which is not overriden in the top classes. This feature is needed because it allows you to roll back to your original notes in case you needed to.

This would also allow creating a "Anki <-> Obsidian" translation method.

logseq-furigana/src/parsers/CommonParser.tsx

Lines 7 to 9 in 7d811a5

    
           fromHtml(content: string) : string | null { 
        
               throw new Error('Not Implemented'); 
        
           }

	fromHtml(content: string) : string \| null {
	throw new Error('Not Implemented');
	}