Preserve Alignment When Converting to Markdown

### Description
Currently, when I align text in the Lexical editor (e.g., center or right alignment) and then switch to Markdown mode, the alignment is lost. This creates inconsistencies when toggling between rich text and Markdown views.

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### Problem
Markdown does not support native text alignment, but many use cases still require alignment (especially for presentations, documents, blog posts, etc.). Losing alignment on mode switch leads to a degraded user experience.

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### Suggestion
Introduce a fallback to preserve alignment in Markdown mode, such as:

#### HTML fallback (commonly used):
```html

**OR Custom syntax (for cleaner round-trip conversion):**
```->This text is center-aligned.<-```
```<-This text is left-aligned.<-```
```->This text is right-aligned.->```