[Feature] Wrap the legend across multiple lines

**Feature description**
When a lot of plots are on the same graph, the legend becomes extremely wide, it would be nice to be able to have this wrap across more lines.

**Feature videos/screenshots**

We are using this as part of the open source Looking Glass project to graph the different stages of the capture and render pipeline to make it easy to diagnose regressions.

The issue does not name files or tests. Start by locating the legend layout and rendering entry points in ImPlot, then reproduce the problem with many plots and compare the current single-line legend with a wrapped layout. Done means legends can occupy multiple lines without making a graph excessively wide.

Written by the indexing model from the issue text.