fix: non-spec emphasis (fix #1806 #2708)

[kshnurov]

#2572 introduced a dirty fix for a very rare non-spec emphasis and lead to many regular emphasis being replaced with html tags, which is no good (#1806 #2708).
This PR fixes that by making sure only non-compliant emphasis will be replaced with html tags.

Please check if the PR fulfills these requirements

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• It does not introduce a breaking change or has a description of the breaking change

[jwlee1108]

@jajugoguma check, please.
@kshnurov Thank you for your contribution. It will take a long time to check this because we are working on other projects. we'll check this PR by mid-October.

[kshnurov]

After doing some testing I've found some very common cases that are completely broken either in master:

    Expected: "test ***test another* test**"
    Received: "test ***test another* test</strong>"

    Expected: "test **test [some](link)** test"
    Received: "test <strong>test [some](link)** test"

or in this PR:

    Expected: "test **test [some](link).** test"
    Received: "test **test [some](link).</strong> test"

Looking at how this editor works with marks and opening/closing tags I don't think it's possible to fix that without a major rework.
I suggest reverting #2572 completely and ignore the rare cases it was introduced for, and here's why:

1. This is a WYSIWYG editor, it's output will be used on the web, which means it will be converted to HTML and that HTML will be valid. There's much more broken things with fix: convert by using HTML tag when it is not between spaces #2572 in place than with having non-spec markdown in some very rare cases (which will be rendered correctly anyway).
2. I've checked how CKEditor works with such non-spec emphasis - it ignores them, i.e. produces non-spec markdown but valid HTML.

@jwlee1108 @jajugoguma what do you think? If you agree, please revert #2572 and release a new version.

[kshnurov]

@jwlee1108 @jajugoguma it's been a month, how's it going? :)

[kshnurov]

@jwlee1108 @jajugoguma 2 months. There's 200+ issues labeled as bug. Shouldn't this repo be marked as unmaintained, so people won't rely on it in production?

[jpradelle]

I think @kshnurov is right, #2572 should be reverted.
I think having HTML in markdown output is an issue, or at least it would be great to have a strict markdown mode without any HTML in it.

[kshnurov]

Resolved in #2754