Refactored var to let or const, used strict equality operator

[grgprarup]

While working with the markdown plugin, I've seen some semantic errors so fixed them. The changes are done in markdown plugin only.
The changes made in this PR:

• Refactored to replace var declarations with let or const for better variable management
• Refactored the code to replace == with === and != with !==

[hakimel]

Thanks!

[t-fritsch]

great work !
@hakimel shouldn't the plugin/markdown/markdown.js and plugin/markdown/markdown.esm.js files get updated too (or maybe it is part of a github action/git hook somewhere) ?

[hakimel]

@t-fritsch Thanks for the reminder, I just recompiled them.

[t-fritsch]

you're welcome ! I wasn't sure this was the way 😅