add hard line break when backslash at EOL

[aduh95]

A backslash at the end of the line is a hard line break.

Ref: https://spec.commonmark.org/0.28/#example-293
Ref: https://spec.commonmark.org/0.28/#example-606
Ref: https://spec.commonmark.org/0.28/#example-609
Ref: https://spec.commonmark.org/0.28/#example-611

Marked version: master#b67b5fd2fcf9fd93e26ffbf10e20455803089467

Markdown flavor: CommonMark

Description

line 1  
line 2\
line 3

Expectation

<p>line 1<br>
line 2<br>
line 3</p>

Result on commonmark.js demo

Result

<p>line 1<br>line 2\
line 3</p>

Result on Marked Demo

What was attempted

I added the backslash rule to the br RegEx.

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

[styfle]

Thanks!