Use grouped bar plots for PkgBenchmark stats

[dpo]

A bar plot should be easier to read than
a line plot.

[dpo]

@MaxenceGollier What do you think?

[Copilot]

Pull request overview

This PR replaces line plots with grouped bar plots when visualizing PkgBenchmark comparison stats (commit vs. reference), making the benchmark results easier to read. The StatsPlots package is added as a new dependency to provide the groupedbar function.

Changes:

• Replaced plot with groupedbar from StatsPlots for benchmark comparison charts, swapping linewidth for bar_width/bar_position parameters.
• Added StatsPlots as a dependency in both using statements and Project.toml (with compat bound 0.15).

Reviewed changes

Copilot reviewed 1 out of 2 changed files in this pull request and generated no comments.

File	Description
src/JSOBenchmarks.jl	Added using StatsPlots; switched plot(...) call to groupedbar(...) with appropriate bar chart parameters.
Project.toml	Added StatsPlots UUID and compat entry (0.15).

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[MaxenceGollier]

Maybe we can open another PR on RegularizedOptimization.jl to see how this looks ?
Not having any unit tests is really not ideal...