application.yml / properties: mark occurrences of ${} property references

This a follow up ticket to https://github.com/spring-projects/sts4/issues/427

More specifically point number 1 for this comment:

https://github.com/spring-projects/sts4/issues/427#issuecomment-625378951

I beleave what is being asked for is a behavior similar to how Java language server implements 'mark occurrences' functionality when you select a variable name in the code.

I.e as show in screenshot below:

However in our case it would instead do something similar when a property reference inside of a '${...}' is selected.

Example:

spring:
  application:
    name: foo
other:
   property: ${spring.application.name}

We place cursor inside the ${...} and it should mark 'name' as a 'occurrence'.

Since Java language server already does this, I think it shouldn't be too hard for us to do the same using existing LSP support.

Similar functionality could also be implemented for 'application.properties' as well.

Start by tracing the existing LSP mark-occurrences behavior and comparing it with the Java language server behavior described in the issue. Verify how application.yml property references are represented, then assess equivalent support for application.properties. Done means selecting a property inside ${...} marks the referenced property occurrence in both formats.