Add Base.active_manifest()

[ChrisRackauckas]

Closes #57924

This PR adds a new Base.active_manifest() function that returns the path of the active manifest file, similar to how Base.active_project() returns the active project file path.

julia> Base.active_project()
"/Users/ian/.julia/environments/v1.13/Project.toml"

julia> Base.active_manifest()
"/Users/ian/.julia/environments/v1.13/Manifest.toml"

(@v1.13) pkg> activate --temp
  Activating new project at `/var/folders/1z/jf841bdj73bdj3vk7kc7f_3w0000gn/T/jl_SCVFBB`

julia> Base.active_project()
"/var/folders/1z/jf841bdj73bdj3vk7kc7f_3w0000gn/T/jl_SCVFBB/Project.toml"

julia> Base.active_manifest()

julia>

The function also accepts an optional project_file argument to return the manifest that would be used for a given project file.

Rebased version of #57937

🤖 Generated with Claude Code

Co-Authored-By: Claude [email protected]

[ChrisRackauckas]

It went beyond the "forget me not"