lang/gleam: new port

Gleam is a fast, friendly, and functional language for building
type-safe, scalable systems. The Gleam compiler itself is written in
rust.

Gleam uses the BEAM runtime, and using the same actor-based multi-core
concurrency, with zero runtime overhead and full inter-operability with
Erlang, Elixir, and LFE.

It has all the features you'd expect from an ML derived language,
including algebraic data-types, immutable data structures, full type
inference, fast compilation, generics, no nulls nor exceptions, and a
few bonus features such as helpful error messages.

Sponsored by: SkunkWerks, GmbH
Differential Revision: https://reviews.freebsd.org/D28795