Fix emphasis with underscore inside word

[mccraveiro]

Marked version: master branch

Markdown flavor: CommonMark

Description

• Related to Defibrillator challenge #1216

Input

пристаням_стремятся_

Expectation

<p>пристаням_стремятся_</p>

Result

<p>пристаням<em>стремятся</em></p>

What was attempted

Fix emphasis with underscore inside word by implementing "left flanking delimiter"

Contributor

• Test(s) exist to ensure functionality and minimize regression (if no tests added, list tests covering this PR); or,
• no tests required for this PR.
• If submitting new feature, it has been documented in the appropriate places.

Committer

In most cases, this should be a different person than the contributor.

• Draft GitHub release notes have been updated.
• CI is green (no forced merge required).
• Merge PR

[mccraveiro]

Regex lookbehind assertions are not available on node v4 😞
By the way, shouldn't we move to node v6 as node v4 end-of-life was on April?

https://github.com/nodejs/Release#end-of-life-releases

[UziTech]

I'm ok with removing node v4 support.

[mccraveiro]

It seems that Regex Lookbehind is behind a flag on node v6.
I'll try to solve this problem with another regex pattern.

[mccraveiro]

@UziTech By the time being I've moved the last 2 commits to another PR so we can move on #1400

[mccraveiro]

It's way harder to implement without lookbehind assertions. I'll close this PR for now.