[cli] add boilerplate theme in command using exports as vehicle

[btopro]

#2004 allows for all the relevant pieces to be exported from here. Most likely we'll want some kind of "hey it's all the stuff you want.js" that exports all kind of things the user would want in building a base theme.

This is just for "let me mod the css / html / basic js and get going" but isn't the solution for a longer term thing. We might want a specific build routine that ignores certain file paths or use an importmap to correctly resolve. Right now the big themes have access to all the powerful elements like site-menu and what not. We may want to make all those available as well or they might "justwork" via the autoloader tho I dont believe so since they are in shadows...