Support Block Chomping Indicator in syntax highlighter

[stevehobbsdev]

When opening a *.fga.yaml where the model DSL contains a Block Chomping Indicator (part of the YAML 1.2 specification), the syntax highlighter for the model DSL does not work and falls back to default colouring.

Example with the indicator |+:

Example without:

This was discovered when testing the forthcoming Export Store command, where the YAML marshaller we use exports the model key using this block chomping indicator to control how new lines and trailing empty lines are treated.