Jake Hillion 55463a9806 fmt: add swift formatting ...

Swift code currently has no auto formatting. Add `swift-format` to the
`treefmt-nix` config to get this formatted.

As our existing Swift code uses 4-space formatting instead of the
default 2-space, also adds a custom `.swift-format

Test plan:
- CI

2026-01-08 13:34:45 +00:00

..

EXOTests.swift

fmt: add swift formatting

2026-01-08 13:34:45 +00:00