Deprecate project_version macro

[GunnarFarneback]

• Document @project_version as deprecated since there is no way to use it correctly short of providing a literal absolute path. The problem is that the result depends on the user's current directory, which doesn't necessarily have any relation to the location of the code using ArgParse.
• Export the project_version function, which can be used correctly in conjunction with @__DIR__. Extend it to handle a directory input.
• Allow the version argument of the ArgParseSettings constructor to take a VersionNumber, to better support the pkgversion function.

Footnote: With ArgParse 1.2, the following constructions can be used in place of @project_version, the last one on Julia 1.9+ in a package.

    ArgParseSettings(add_version = true, version = ArgParse.project_version(Base.current_project(@__DIR__)))
    ArgParseSettings(add_version = true, version = ArgParse.project_version(joinpath(@__DIR__, "..", "Project.toml")))
    ArgParseSettings(add_version = true, version = string(pkgversion(@__MODULE__)))