[typespec-java] Remove typespec-extension directory and flatten the folder structure #4759

for consitency how much work would it be to not have this as a sub folder? the fact there is a java code base underneath is an implementation detail of the emitter. Would be good to have at least the package.json at the root of the package.

Originally posted by Timothee Guerin (@timotheeguerin) in #4591 (comment)