transpile-only tasks for client and extensions
* extract transpile into its own file
* add transpile-client task, polish transpiler
* add transpile-extensions, improve transpile logic
* move declaration of "const enum" above it usage so that it can be used with const-enum-inlining
* (ugly) make d.ts transpilation configurable because it is needed for extensions but a problem for client
* hack my way around so that `getOwnEmitOutputFilePath` is reusable by our transpile
* honor `noEmit` flag