Consider deprecating/removing `contentlayer dev` in favour of `contentlayer build --watch`

[schickling]

[seancdavis]

Suggestion: Just contentlayer and contentlayer --watch or contentlayer --dev?

Justification is that tools like Next are multi-purposed and provide a better experience using commands for particular actions — build, export, dev, etc.

Libs like Jest have one primary function (to run a test suite) attributed to the CLI command (jest). It then uses flags to deviate behavior from the norm (jest --init).

I like the latter pattern for this project. It also then leaves open the possibility that the first argument could be used for something else down the road.