Cleanup, more CI testing for scripts, validate inputs

[stertooy]

• Turn more steps into separate scripts that are now tested in CI (see Split out some of the more complex steps into separate files and add tests #27)
• Move all scripts to a folder.
• Replace GAP Error by Exec("echo \"::error:: [...] \""); FORCE_QUIT_GAP(1); to make the errors more visible in GitHub Action logs.
• Validate inputs when relevant (like in run-pkg-tests).
• Made indendation consistent (2 spaces per level).