Adjust codegen to allow custom mainargs.TokensReaders and exercise it via example tests for Bundle Libraries docs

[lihaoyi]

• Includes a mainargs.TokensReader[os.Path] by default for convenience, and added example tests for 4 of the listed bundled libraries
• Removed the type parameter from Discover[T], since it was not necessary and only ever caused type inference problems
• Moved the lazy val millDiscover = Discover[T] call from the MillMiscInfo object to the main body of the class containing user code, so it can see anything the user writes in build.mill, including TokensReader instances. The old reasoning for having it outside (that we want it to propagate implicitly to be picked up by nested RootModules) no longer applies with the codegen-time RootModule unpacking

Fixes #2029
Fixes #2857