Code loading: add support for "portable scripts".
These are jl files that have project and manifest embedded inside them, similar to how Pluto notebooks have done for quite a while.
The delimiters are `#!manifest begin` and `#!manifest end` (analogous for project data) and the content can either have single line comments or be inside a multi line comment.
When the active project is set to a portable script the project and manifest data will be read from the inlined toml data.
Starting julia with a file (`julia file.jl`) will set the file as the active project if julia detects that it is a portable script.
