Support multiple expressions in code blocks

[istathar]

Update the paresr of CodeBlocks to support multiple expressions (separated by newlines) and expressions sequenced on a single line (separated by ; characters).

Element::CodeBlock now holds Vec instead of single Expression.

New Expression::Separator variant, representing ; between expressions in a code block, capturing whether the document author used semicolons.

read_code_block() updated to return a Vec instead of a single Expression. This is done by looping over reading expressions until the block is consumed. After each expression it consumes an optional ; and pushes Expression::Separator if one is found. Errors on an empty block. The old leftover-detection heuristic is removed entirely.

Formatter updated accordingly.

[istathar]

Here's the example that was used while developing this

I do wonder how we should treat enum-like values that appear for example in navigate() here but that's a language design question for another day.