HTML mode: Map zero-width characters to their entity codes.

[diegoreymendez]

This issue was spawned from the discussion here.

We should make sure that whenever the user switches to HTML mode, any zero-width character is converted to its entity code (&#xfeff).

This is because the character is invisible, so adding the code lets the user clearly see that it's there.

[salty-horse]

Here are some other zero-width characters listed on Wikipedia:

• U+180E mongolian vowel separator
• U+200B zero width space - ​
• U+200C zero width non-joiner - ‌
• U+200D zero width joiner - ‍
• U+2060 word joiner - ⁠

[diegoreymendez]

Updated the title and description to apply to all zero-width characters.

[salty-horse]

U+200D is tricky, as it's commonly used to combine emoji. 👨‍❤️‍💋‍👨 will appear as 👨‍❤️‍💋‍👨