Define custom Linestyle type (fix #2851)

[Datseris]

This PR defines the new Linestyle type for handling custom linestyles. It also fixes #2851 already in its current state. What I don't know:

• Where to put export Linestyle.
• Where to put this Linestyle definition.
• Where to put the deprecated convert method.
• How to test this.

Any help is appreciated thank you all!

[ffreyer]

• Where to put export Linestyle.

Makie.jl

• Where to put this Linestyle definition.

We usually have structs with their functions if we can, so conversions.jl is probably fine. Though maybe there is a point in putting it in Makiecore/types.jl, so it's available for packages that want to extend Makie without depending on Makie directly.

• Where to put the deprecated convert method.

We haven't consistently put them in deprecated.jl but I think that would be best to keep track of deprecations.

• How to test this.

As far as I can tell the convert_attribute pipeline is pretty short, starting from

Makie.convert_attribute(value, Makie.Key{:linestyle}(), Makie.Key{:lines}())

For testing I would call that with different values and check if they get converted correctly.

[SimonDanisch]

Continued in #3193

[Datseris]

ops, i am sorry, i jave forgotten this one, thanks a lot for taking over@!!!