New & improved string diff viewer

[boatbomber]

Closes #968.

New Features:

• Diffs now highlight the specific characters that changed, not just the entire line
• Removed/Added lines create spacer lines on the other side, keeping both sides aligned
• Scrolling position of both sides is linked, making it easy to scroll
• Added line numbers on the side
• The scroll bar has red/green markers to help you quickly navigate to diff areas
• The diff viewer will open up already scrolled down to the first diffed line to save you the time

Fixes/Improvements:

• Long scripts no longer glitch out the text rendering
• Rendered as a virtualized list, so long scripts won't be creating a million text labels now