yeoman-generator wants conflicting transitive dependencies on mem-fs

[ntucker]

Fix:

Update @yeoman/types mem-fs dependency to 4

Problem:

yeoman-generator depends on @yeoman/types and mem-fs@4. But @yeoman/types doesn't like mem-fs@4 - it wants @3.

mem-fs is listed by your project with version 4.0.0, which doesn't satisfy what @yeoman/types (pac28d) requests (^3.0.0).