[Markdown] _Italic_ is rendered as underlined instead of italic

### Expected
Two underscores surrounding a word should result in the word being italic in preview.
This is so in Electron, iOS and for same note published on the Web.

### Observed
In preview, it's displayed an underlined:

https://user-images.githubusercontent.com/73365754/107771223-1c4d5a80-6d43-11eb-94a3-f00fcc9cddab.mov

### Reproduced
1. Open new note
2. Enable MD
3. Enter `_Italic_` text (note that it's even shown as italic in edit mode)
4. Open Preview ~> it's underlined instead of italic.

Make|Model|macOS Version|App Version
-|-|-|-
Mac|MBP 16"|11.2|2.8 beta 3

Start by reproducing the issue in the macOS app's Markdown Preview using the listed steps and `_Italic_` input. Trace the Preview rendering path from the note editor to the Markdown renderer, then verify that the text is italic rather than underlined and that existing Electron, iOS, and Web behavior remains consistent.

Written by the indexing model from the issue text.