WithPre refactor

[brendt]

The current WithPre interface has a number of problems:

• It's an awkward name, I want to come up with a better one
• Maybe it's better to get rid of this interface altogether and have all themes return their wrapping tags that can be used within a web context
• Themes have no inclination about the current language, making it impossible to add the data-language attribute (Adds data-lang attribute for CommonMark code block renderer #90) to the pre tags

My proposal:

• Get rid of WithPre
• Introduce an interface WebTheme, similar to TerminalTheme, but this one requires implementations for preBefore and preAfter
• Find better names for preBefore and preAfter
• Inject the highlighter into these two methods, so that we can access the current language (and other data if needed)
• With the addition of WebTheme, we can consider moving escape logic to the theme itself instead of managing it within the highlighter.

[brendt]

@aidan-casey what are your thoughts on this idea?