feat(analysis/inner_product_space/two_dim): new file (#16928)
This file defines constructions specific to the geometry of an oriented two-dimensional real inner product space. Main declarations:
* `orientation.area_form`
* `orientation.right_angle_rotation`
* `orientation.kahler` (renaming suggestions are welcome)
[Zulip](https://leanprover.zulipchat.com/#narrow/stream/217875-Is-there-code-for-X.3F/topic/adding.20angles/near/299839950)
