[opinionated] Prefer emitting `-` over `*` for unordered list markup

[zm-cttae]

⚡️️ Feature Request

Problem statement

• As a: open-source developer writing issues on GitHub
• I want to: use the same modern Markdown settings as my organisation or repository
• So that: I can onboard and share comments consistent to a Markdown style guide.

Proposed solution

Adopt the GitHub and CommonMark conventions of Markdown formatting punctuation/operators.
In an ideal world the editor would emit - for unordered lists.
Other settings are { headingStyle: 'atx', bulletListMarker: '-', emDelimiter: '*', br: '\n' }
Technically, this is a breaking change to the project and may not be worth the dev time.

Describe alternatives you've considered

A setting is provided so that users can select their preference.
This is more complex and requires a big growth to the the codebase.
Honestly why would someone intentionally want to use * and setext headings like ## Header 2 ##.
It seems secondary to having some saner defaults.

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[zm-cttae]

This has nothing to do with github writer - it's a upstream issue in turndown. mixmark-io/turndown#424

[zm-cttae]

Reopening in case the upstream PR is rejected