Add build-eslint-rules to npm prepare script

[andrewbranch]

My expectation is that my editor should be able to function properly in the TypeScript codebase after npm install && npm run build. This ensures custom eslint rules are built as part of the latter former command to avoid getting this:

[weswigham]

Why not make it a prepare script rather than part of the main build? Running lint on the command line has building the lint rules as a dep already, and they don't change all that much, so running the build once immediately after install should be fine.

[andrewbranch]

@weswigham that works, done