use 'files' instead of .npmignore

[midgleyc]

What is the purpose of this pull request?

• Documentation update
• Bug fix
• New rule
• Changes an existing rule
• Add autofixing to a rule
• Other: build simplification

What changes did you make? (Give an overview)

Change build to use files over .npmignore.

.npmignore lists the files that should not be in the build, and can easily go out of date (and has) because it needs updating every time a file is added that shouldn't be in the build.

files in package.json lists the files that should be in the build, and can be kept up to date more easily.

I include the rules folder, and the files which are always included automatically.