Mention build tools other than sbt.

[dubinsky]

Scala.js documentation uses sbt build tool exclusively (e.g., in https://www.scala-js.org/doc/project/); I think other build tools that support Scala.js should be at least mentioned:

• scala-cli
• Mill
• Gradle with Scala.js / Scala Native plugin

Thank you!

[sjrd]

PR welcome.

[dubinsky]

PR welcome.

Here you go: #658.

Thank you!