Fix ES|QL docs railroad diagrams comma positions for optional parameters (elastic#134298)

The SVG images in ES|QL docs had many mistakes with comma positions inside optional parameter groups.

For example, the bucket command does not include the comma in the optional group, making it look like we need to type a trailing comma. Almost twenty images are fixed, while also taking care to handle commas at the end of trailing optional parameters, and complex cases like COALESCE.

We also added unit tests for the RailRoadDiagram class to make it much easier to test and debug this, as well as reducing the chance that further refinements break anything.

Author: craigtaverner