Add `netlify serve` command

[ascorbic]

Is your feature request related to a problem? Please describe.

Currently the dev command is used to run the framework's dev command, but this is usually not the same as the production site.

Describe the solution you'd like

A serve command that does the following:

• By default, runs the build command and uses the config modified by the build plugins. This can be disabled with --no-build
• Runs the static server and functions server, in the same way as --framework=#static

Describe alternatives you've considered

Running ntl deploy --build, killing the process once the modified toml has been created, then running ntl dev --framework=#static

Additional context

Would work well alongside #3906, in which case it could skip building by default and instead have a --build flag like deploy.

Can you submit a pull request?

Yes

Pull requests are welcome! If you would like to help us add this feature, please check our contributions guidelines.

[erezrokah]

Thanks @ascorbic, to add to it, we'll probably want to re-run build on changes to source files.

[jdahdah]

To add a use-case here, there is currently no way to locally test if Netlify redirects are working correctly when using gatsby-plugin-netlify, as those are only generated at build time. Having this would be really helpful for debugging!