lang/gleam: update to 0.21.0

Now with LSP (Language Server) support

• https://gleam.run/news/v0.21-introducing-the-gleam-language-server/
• https://github.com/gleam-lang/gleam/blob/main/CHANGELOG.md#v0210---2022-04-24

Sponsored by: SkunkWerks, GmbH