Move traits to ProximalCore.jl & implement is_locally_smooth where suitable

[hakkelt]

Besides removing traits.jl that contained the functions moved to ProximalCore.jl and applying renaming that occurred in ProximalCore.jl v0.2.0, the commit contains the following changes:

Added [workspace] entry to Project.toml. It's a Julia v1.12 feature, ignored by previous versions, that synchronizes versions between subprojects test, docs, and benchmark.

Added traits:

• is_proximable = true added to SqrNormL2
• is_positively_homogeneous = true added to SumPositive
• is_locally_smooth = true added to LogBarrier
• is_locally_smooth = true added to NormL2

is_locally_smooth(T{W}) = is_locally_smooth(W) added where W is the wrapped function:

• Postcompose
• Precompose
• PrecomposeDiagonal
• Regularized
• SeparableSum
• SlicedSeparableSum
• Sum
• Tilt
• Translate

Implements part of #151