Add option to customize Mill exported project name

[carlosedp]

This PR adds an option to export command allowing customization of Mill project name (currently assumed as "project").

This changes:

• Mill project name on build.sc (object myprojectname extends ScalaModule {)
• Mill project directory is named to the flag (./dest/myprojname/src/...)

[Gedochao]

Hey, thanks for contributing!
Since you've added a new option, you need to also update the relevant reference doc to make the associated test pass on the CI.
You can do it automatically by running:

./mill -i generate-reference-doc.run

It seems there are conflicts with other recent changes as well, so make sure to rebase the branch while you're at it.

[Gedochao]

Can you also apply this new option in the integration test?
The current shape of our tests assume the Mill project name to always be project.

scala-cli/modules/integration/src/test/scala/scala/cli/integration/ExportMillTestDefinitions.scala

Line 55 in f470e88

val projectName = "project"

[carlosedp]

Added docs and test case.