import.less instead of lessimport

Currently LESS users can define some `.lessimport` file to indicate that the file shouldn't be automatically added in the css ressources but is only here to be imported in another LESS stylesheet.

* Meteor is the only project that use this `.lessimport` extension. [[1]](https://github.com/search?p=1&q=lessimport&ref=cmdform&type=Code)
* Most text editors support syntax hightlighting based on the file extension. As a result `.lessimport` files are not recognized as less files, and there is no syntax hightlighting by default.

I suggest to use the `.import.less` extension. The implementation and behavior are exactly the same, but those files will be recognized as less files in most environments.