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

[kdvolder]

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.