fix: support compose.yml / compose.yaml as default compose file names

[kmplngj]

mocker compose only resolved docker-compose.yml as the default config file, ignoring the preferred filenames introduced in Docker Compose V2.

Changes

• ComposeFile — adds defaultFileNames (ordered: compose.yaml, compose.yml, docker-compose.yaml, docker-compose.yml) and findDefault(in:) which returns the first match in a given directory, mirroring Docker Compose V2 lookup behaviour
• ComposeOptions.file — changed from String = "docker-compose.yml" to String?; resolves via findDefault() when -f is not passed; error message now lists all searched filenames
• Tests — 5 new cases covering priority order (compose.yaml > compose.yml > docker-compose.yaml > docker-compose.yml), fallback, and nil when none found

# Previously failed — now works
$ ls compose.yml
compose.yml
$ mocker compose up
[+] Running …

[us]

thank you great catch! @kmplngj