Make Mill -native launcher the default in docs and examples

[lihaoyi]

This would let everyone benefit from the improved responsiveness of the native launcher by default, and remove the "need a JVM installed" dependency that should simplify bootstrapping and help Mill adoption in non-JVM projects where such an install may be annoying

Presumably "making it the default" would mean 0.13.0 would default to the native launcher, and we would use a different suffix (e.g. 0.13.0-jvm?) for the JVM-based client/launcher

This should only happen in 0.13.0, after the Mill native launcher has had a bit more time to bake and be dogfooded in 0.12.6. Also need #4414 to land first.