Horizontal and vertical splitter

Splits frame in half with access to each frame by two lambdas for each half

**Is your feature request related to a problem? Please describe.**

Not being able to place elements in one line with different directions

**Describe the solution you'd like**

Horizontal and vertical splitter that splits frame in half

**Describe alternatives you've considered**

-

**Additional context**

Couldn't think of a way to split by n times with access to different frames at once, there is a way to make a different function for each n way split, but it would be messy for sure

The issue names no files, tests, or entry points. Start by locating egui's frame and layout APIs and existing horizontal or vertical layout examples, then clarify the callback shape and split semantics with maintainers. Done means a defined splitter API with documented behavior and coverage for both orientations.

Written by the indexing model from the issue text.