Bump railroad-dependency

[lukaslueg]

This bumps the railroad dependency used to draw the syntax diagram to the latest version, and changes the rendered diagrams in three particular ways:

• ExpressionKind::NegativeLookahead used to be rendered "inline", which was counterintuitive (the rendered part is what not to read). It was replaced with upstream's Negative Lookahead Assertion.
• The Reference-specific Except-Box used to render exceptions specifically to CHAR Non-Terminals was replaced by upstream's Negative Lookbehind Assertion.
• Repetitions used to be rendered with a single element; replaced with the proper diagram node.

Upstream changes include some minor bug fixes around corner cases, which the Reference didn't use to hit afaics.

I built and visually inspected the reference locally, and it seems fine to me.

Note that there are some uses of comments to denote "except this", "must be followed by" etc.. I purposefully left these alone.