Update to support gleam_stdlib > 0.47.0

[chrissimon-au]

In 0.48.0, gleam_stdlib removed gleam/string_builder which had been deprecated since 0.42.0 in favour of gleam/string_tree.

On a fresh install, I ended up with gleam_stdlib@0.51.0 and glacier@1.1.0 and tests failed with:

error: Unknown module
  ┌─ <project path>/build/packages/glacier/src/glacier.gleam:4:1
  │
4 │ import gleam/string_builder
  │ ^^^^^^^^^^^^^^^^^^^^^^^^^^^

No module has been found with the name `gleam/string_builder`

Workaround

Can workaround by pinning gleam_stdlib to 0.47.0 in gleam.toml but it would be great to be able to use latest stdlib and glacier together.

[chrissimon-au]

See #17