Syntax Highlighting in Manual

A small update to the Sprite32! Manual to add syntax highlighting for FREMScript code. 

During initial development, I experimented with various approaches to highlighting the manual's syntax using the Godot TextEdit control (the same used for the code editor), but couldn't quite eek out a theme I liked. For the first few releases, I opted to keep it simple: plain text without syntax highlighting.

Over the past few coding sessions, I've revisited the theme of the manual and have worked out something that should be much more readable. Code examples should stand out much better now! 

One of the key challenges was coming up with a syntax highlighting theme that worked well on a lighter background, since I wanted the manual to feel more like something printed on aged white paper. Some lighter-colored symbols like strings, comments, and parentheses work well on the editor's dark background, but not so well on the manual's off-white background. With a little experimentation and some custom font overrides, I was finally able to get something that worked well for the manual.

During your next Sprite32! coding session, be sure to mash that F1 key to check out the new theme.