Rename @scriptdir to @script, and look for environments in parent directories

[tecosaur]

In today's triage call, Stefan's comment about @scriptdir https://github.com/JuliaLang/julia/pull/51149#issuecomment-1938993314 (introduced in #50864) was discussed a bit further, and triage seemed broadly approving of the idea that:

• @scriptdir be renamed to @script
• @script behaves like @., in that it automatically searches up parent directories for the first environment found

These changes (specifically the first one) will need to be made before 1.11 to be non-breaking.