Improve "building" of Mill bootstrap scripts

[lefou]

Currently, the various start files are used as-is. They are cluttered in different locations and must be edited manually, which is error-prone and cumbersome.

To properly finish up #2161 and esp. comment #2161 (comment), we should do the following:

• Move scripts (mill, mill.bat, ci/mill.ps1) into new dir scripts (or scripts/src)
• Replace all movable parts with placeholders (script version, default versions, links to web-site and download locations)
• Add a Mill build step to build final script from these templates
• Consolidate further testing under the scripts project
• Automate, when the <project-root>/mill{.bat} scripts need updates and fail CI or autofix it.