fix: return early from ftl dev when incompatible flags (--stop --no-serve) are used

[deniseli]

Fixes #1167

Manually tested:

$ ftl dev --stop --no-serve ./examples/go --log-level=trace
trace: Loading config from /Users/dli/Development/ftl/ftl-project.toml
debug: FTL serve is not running in the background
$ ftl serve --background
info: `ftl serve` running in background with pid: 42320
$ ftl dev --stop --no-serve ./examples/go --log-level=trace
trace: Loading config from /Users/dli/Development/ftl/ftl-project.toml
info: `ftl serve` stopped (pid: 42320)