Move LambertW to SpecialFunctions

[jlapeyre]

There is an outstanding request to move LambertW to SpecialFunctions.
Someone needs to do this and issue a PR. Maybe me.

[jlapeyre]

I made a PR adding LambertW to SpecialFunctions.jl seven days ago. Waiting for a response.

[jlapeyre]

I will close the PR and make a new one after v1.0 is released.

[caseykneale]

Hey, the Lambert W Fn is really useful! Can we get it merged into SpecialFunctions? :D @jlapeyre

[jlapeyre]

Good idea. I am really busy now. PRs welcome :)

[nalimilan]

x-ref JuliaMath/SpecialFunctions.jl#84