[Typescript] Add RouterSource typings and remove typing dependency to switch-path

[JabX]

Hello,

I tried to setup a Cycle project using Typescript and Cyclic-router but I ran into two issues:

• makeRouterDriver's typings references RouterSource's, that in turn references switch-path's that aren't included in the build. Thus a typescript app that includes makeRouterDriver cannot compile, which is a problem.
• The RouterSource typings aren't exposed to consumers, so they can't type their Sources objects. They can be accessed "manually" though, but they're incomplete because they have to work with multiple stream libraires.

With this PR, I attempt to solve these two issues:

• I added an interfaces.ts files and moved there the two interfaces that were previously in switch-path's typings, so that the resulted .d.ts files do not depend on switch-path. Ideally, adding typings to switch-path could also have solved the problem, but in the meantime we have a solution that doesn't depend on that.
• I added 4 root .d.ts files containing typings for RouterSource for all supported stream librairies, exactly like @cycle/dom does.

[TylorS]

This is awesome! Thank you very much LGTM

[TylorS]

Published as 2.1.2