Inline (YAML) Langauge Injection in JetBrains IDEs

JetBrains IDEs (IntelliJ, GoLand, etc) have a nifty feature called Language Injection that lets you get full language features when a language is embedded within another. For example, a SQL query within a string within a Go file. A few of these come out of the box, but they are pretty limited -- I only had some XML ones prior to enabling the Databases plugin which added a few SQL ones. Fortunately, there is the ability to add custom ones. Unfortunately, this is expressed in a proprietary language with, as far as I can tell, zero documentation.