email build/start should have a --port option to write the port into package.json or to start the build on custom port

[ADTC]

Goals

Neither email build nor email start has a --port option, which means we cannot use the CLI to set a port the build should start on.

Background

Port 3000 may be already used by another service. Currently you have to manually edit package.json to change the start command to set a different port: next start -p 4000

Related but different discussion:

• `react-email` should automatically use the next available port #941

Proposal

Add the --port option to email build so that it can write the port into the start command inside package.json of the build output.

If possible, also add the --port option to email start so that a different port can be dynamically used rather than 3000 or the one set it package.json.