[WPB-28448] Add headroom to treefmt, pin to 0.5.0.0 (github master).

[fisx]

https://wearezeta.atlassian.net/browse/WPB-28448

Checklist

• Add a new entry in an appropriate subdirectory of changelog.d
• Read and follow the PR guidelines

[fisx]

Headroom tests are failing. Maybe we should wait for a more stable commit (or provide one)?

headroom> Failures:
headroom> 
headroom>   test/Headroom/CLISpec.hs:27:9:
headroom>   1) Headroom.CLI, headroom CLI, prints a structured error for invalid project configuration
headroom>        uncaught exception: IOException of type NoSuchThing
headroom>        headroom: readCreateProcessWithExitCode: posix_spawnp: does not exist (No such file or directory)
headroom> 
headroom>   To rerun use: --match "/Headroom.CLI/headroom CLI/prints a structured error for invalid project configuration/" --seed 1334006307
headroom> 
headroom> Randomized with seed 1334006307