Error showing how to use `case` instead of `if`

[tryoxiss]

Related:

• If statement discussion: https://github.com/gleam-lang/gleam/discussions/2158#discussioncomment-10455378
• Reply to comment on making it convert if to case: https://github.com/gleam-lang/gleam/discussions/2158#discussioncomment-10455378

A simple lint which would convert a more conventional if statement into a case expression.

Examples:

if alpha > beta {
  alpha
} else {
  beta
}

this would give a helpful error, explaining that if does not exist and you should use case instead:

error: `if` is invalid
  ┌─ /src/main.gleam:1:1
  │
6 │   if alpha > beta {
  │    ^^

`if` is not valid gleam.

Hint: You can do this with Case instead:
case alpha > beta {
  True -> alpha
  False -> beta
}

See: https://tour.gleam.run/flow-control/case-expressions/

I imagine the algorithum for this is fairly simple. If an if statement is found, extract the expression beginning after it until the curly brace, the code within the if {} block, and the code within the else {} block, and subsitute them here:

case <<CONDITION>> {
  // if its single line you can remove the block
  True -> { <<CODE_IF>> }
  False -> { <<CODE_ELSE>> }
}

This solution only works for boolean expressions, but thats fine because an if is inherently boolean. Im sure the error message itself can be improved a lot too, I just wrote something to get the idea down.