Regex language

[RunDevelopment]

This PR adds a Regex language based on the JavaScript Regex syntax.
Edit: It now supports most of the regex syntax of C# and Java.

This is based on #1468 with a few improvements.

Also, I'm thinking about editing the themes to show the results.

[shreyasminocha]

Thank you so much for this. I believe Prism is the only mainstream syntax highlighter with regex highlighting.

Also, I'm thinking about editing the themes to show the results.

I'm assuming you didn't get around to doing this yet. Meanwhile, I'm using a stylesheet I wrote with colours lifted from RegExr for anyone looking for that.

[RunDevelopment]

I'm assuming you didn't get around to doing this yet.

Thanks for reminding me. I completely forgot about this.
However, I won't change the themes for this as there are quite a few to change. I'll add a few aliases to given them reasonable highlighting which hopefully will look reasonably good in all themes.