LS: code action to inline all constant usages

[ankddev]

Related to #5891 and could be fixed in the same PR. I think that we could offer code action with name like Inline all constant usages on constant definition. For example:

const x = 0
//        ^ Trigger code action here

pub fn main() {
  echo x
  wibble(x)
}

would result in this code:

pub fn main() {
  echo 0
  wibble(0)
}

Open questions

• Should it delete constant definition?
• Should we be able to trigger it only on definition or on usages too?

[lpil]

What would be the purpose of this code action? When would it be beneficial?

[ankddev]

I find it benefitical when I start with unified constant value, but then consider (for example, while refactoring) that I need different values in many places. Then I trigger this code action and change values to desired, sometimes extracting them to constant. There's an alternative way, though: you could call code action to inline constant on each usage that needs different values. This is good, because you can call find references and see all places, where changing could be needed, but can become quite boring, when there are many places to change.