[WIP] Import missing given instance

[tanishiking]

refer to scalameta/metals-feature-requests#141

This PR enables us to import missing given instance

TODO

• parse top-level given
• testing

There're some limitations:

• we don't filter missing instances by type, so if there's stringOrd: Ord[String], we will receive suggestions both for intOrd and stringOrd, I'm wondering can we filter it with a reasonable computation cost 🤔
• if there's no explicit method definition under the given instances
  • for example, we can't find lt and gt because they're not under given instances.

trait Ord[T]:
  def compare(x: T, y: T): Int
  extension (x: T) def lt (y: T): Boolean = compare(x, y) > 0
  extension (x: T) def gt (y: T): Boolean = compare(x, y) < 0

given intOrd: Ord[Int] with
  def compare(x: Int, y: Int): Int = if x < y then 1 else -1

[tanishiking]

I think this PR needs some rework, closing for now