Provide a "script modified" editor-only signal for @tool scripts to automatically "refresh" themselves

[chriswa]

A use case for @tool scripts is in previewing procedural generation. When preview generation is performance-intensive, @tool scripts can avoid unnecessary re-generation by subscribing to various events representing dependant data modifications; however, changes to the @tool script itself (or dependant @tool scripts) are not able to be detected.

Personally, I make changes from VS Code, and I can observe that my @tool script is hot-swapped almost instantly; however, I'm not able to detect when it is hot-swapped, so I must resort to polling.

It would be ideal if (@tool) Script objects supported a modified signal, which @tool scripts could subscribe to.

Workarounds:

• poll the filesystem for the modified date of the script(s),
• require the user to manually click @export_tool_button,
• require the user to manually reload the project — or close and re-open the scene.