Use StencilSwiftKit instead of Stencil

[Liquidsoul]

I use stencils in Sourcery and Swiftgen and I miss some nice additions from StencilSwiftKit.
It would be really cool if SwagGen could use it. This would introduce some nice additions for custom templates like macro, set and some nice string filters.

I saw that you use your own version of Stencil to add some feature to the include node. It may not be feasible but this is at least for your information in case you did not know it existed 😉

[yonaskolb]

Yeah, I'd be up for linking to StencilSwiftKit. As you say though I've added some non compatible customizations to Stencil.
The line escaping would need to be added to Stencil, via my PR or probably this one stencilproject/Stencil#92
Once that's done I can move my include changes into a new tag if it won't get merged

[yonaskolb]

Once stencilproject/Stencil#214 is merged and SwiftStencilKit uses it in a new release we can move to SwiftStencilKit.
A few of us are seeing if we can get access the Stencil repo or make a hard fork of it to speed up updates krzysztofzablocki/Sourcery#567

[yonaskolb]

Implemented in #111