feat(compiler): Introduce warning if non-void expression value not used or ignored

[alex-snezhko]

The below code will now give a warning

let someThing = (value) => {
  if (value) {
    Ok("Good")
  } else {
    Err("Not good")
  }
}

let f = () => {
  someThing(false)
  123
}

[ospencer]

Let's land this in the next breaking so users aren't surprised by the warnings.

[ospencer]

@alex-snezhko can you rebase this? Looks like the tests are failing on merge because of the LSP tests.

[alex-snezhko]

@ospencer rebased and fixed tests

[ospencer]

@alex-snezhko looks like tests are failing because of formatting.

[marcusroberts]

Nice improvement, I like getting these warnings