Use @resolver-engine/imports

refactored file explorer

fixed tests (expanding)

Merge branch 'swap_it_fileExplorer_ref' of https://github.com/ethereum/remix-ide into swap_it_fileExplorer_ref

- fixed bug for empty file during create gist

- landing page restyleing

- fixed bug for empty file during create gist

- landing page restyleing

refactor/simplify code in the tabs

Remix plugin api