fix: Defaulting emphasis input/serialization to _ from *

[tmcentee]

Closes #59 .

Describe your changes

This simply changes the default Markdown serialization for emphasis (italics) from * to _. This now allows text with overlapping marks (e.g. bold and italics) to roundtrip between rich text and markdown successfully.

In rich-text mode, either _ or * can be used to mark text with emphasis. Both will serialize to _ in Markdown.

PR Checklist

• [ ] All new/changed functionality includes unit and (optionally) e2e tests as appropriate
• [ ] All new/changed functions have /** ... */ docs
• [ ] I've added the bug/enhancement and other labels as appropriate

Environment(s) tested

• Device: Desktop
• OS: Win10
• Browser: Chrome
• Version: 103

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[tmcentee]

cc @b-kelly I know you had commented on the reporting Meta post about some edge cases you can think of. Let me know if you still have any in mind that I'll need to check out here.

https://meta.stackexchange.com/questions/360033/opt-in-alpha-test-for-a-new-stacks-editor/360136#comment1202381_360136

[tmcentee]

@b-kelly This has been orphaned for a while. Still on the radar to eventually ship or should I close?