Use `_` as default character for emphasis and `*` for strong text

What does this MR do?

It modifies Toast UI’s HTML to Markdown converter to allow specifying the syntax to make text strong or emphasized when generating Markdown from the WYSIWYG editor. By default, the strong character will be * and the emphasis character will be _ following the Handbook Markdown guide. For example:

**Strong text**
_emphasized text_

How to test?

Open a markdown document in the Static Site Editor. In your local environment, you can open http://127.0.0.1:3000/h5bp/html5-boilerplate/-/sse/master%2FREADME.md. Add a list and a sublist using the editor:

When switching from WYSIWYG mode to Markdown mode or saving the changes, you should see that text.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team