[WIP] Feature/import cost layer

[akinsho]

Was inspired by https://github.com/yardnsm/vim-import-cost to implement an import cost buffer layer similar to this.

It uses the import-cost npm package which does all of the hard work I just render the result of it in a buffer layer.

The layer is in the extensions dir and is something that I think can be separated out once we have plugin management sorted.

Outstanding:

• Attempt to add jest test for layer
• Add configurable size boundaries for packages and colors config opts for sizes
• Allow priorities to be specified for layer i.e. this would clash with git blame but currently has a hard coded z-index (make that user configurable)