text: Enable mixed bold/italic markdown rendering

[Third-Thing]

The markdown preview did not show text correctly when bold and italic were used together. For example, text marked as both bold and italic could lose one of those styles.

The solution keeps the existing text styles when nested markdown is parsed, then adds the outer style to the same text. This lets combined styles, such as bold plus italic, render correctly in the preview. (Closes #2120)

Tested in gpui-component-story --example markdown

_italic_ *italic*
**bold** __bold__
***bold and italic*** ___bold and italic___
_**bold and italic**_ *__bold and italic__*
*italic with **bold*** _italic with __bold___
_italic **with** bold_ *italic __with__ bold*
**bold with *italic*** __bold with _italic___
__bold *with* italic__ **bold _with_ italic**

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Implemented-by: Codex GPT-5.5
Reviewed-by: Claude Opus 4.7 and Human

[madcodelife]

Thanks! I tested the styles inside the markdown reference are still not preserved. Could you handle this in the same PR?