Rename at-scriptdir project argument to at-script

NEWS.md

@@ -68,6 +68,7 @@ difference between defining a `main` function and executing the code directly at
 * The `--compiled-modules` and `--pkgimages` flags can now be set to `existing`, which will
   cause Julia to consider loading existing cache files, but not to create new ones ([#50586]
   and [#52573]).
+* The `--project` argument now accepts `@script` to give a path to a directory with a Project.toml relative to the passed script file. E.g. `--project=@script` for the same directory or `--project=@script/foo` for the `foo` subdirectory. ([#50864] and [#53352])
 
 Multi-threading changes
 -----------------------

base/initdefs.jl

@@ -272,7 +272,7 @@ function load_path_expand(env::AbstractString)::Union{String, Nothing}
         env == "@" && return active_project(false)
         env == "@." && return current_project()
         env == "@stdlib" && return Sys.STDLIB
-        if startswith(env, "@scriptdir")
+        if startswith(env, "@script")
             if @isdefined(PROGRAM_FILE)
                 dir = dirname(PROGRAM_FILE)
             else
@@ -283,7 +283,7 @@ function load_path_expand(env::AbstractString)::Union{String, Nothing}
                 end
                 dir = dirname(ARGS[1])
             end
-            return abspath(replace(env, "@scriptdir" => dir))
+            return abspath(replace(env, :wq"@script" => dir))
         end
         env = replace(env, '#' => VERSION.major, count=1)
         env = replace(env, '#' => VERSION.minor, count=1)