Custom Layout Engine for Fiber Reconciler

[carson-katri]

This adds a layout pass to the WIP fiber reconciler, that should in theory be able to replicate SwiftUI layout 1:1 as it’s developed.

In this example, Text is not measured and is a fixed size of 100x50.

Here are some things that are missing, but should be implemented (perhaps as separate PRs targeting this branch):

• Renderer-specific Text measuring
• alignmentGuide modifier
• Coordinate Spaces
• Disabling layout computations for renderers that don't, or optionally, use them
• Layouts for all of the existing Views and modifiers, such as ZStack, .background, .overlay, .frame etc.

[carson-katri]

I've made some revisions, and it now seems to work well with simple examples:

Screen.Recording.2022-03-03.at.10.49.13.AM.mov

[MaxDesiatov]

A few more formatting and doc comments nits

[MaxDesiatov]

Perfect, thanks!