feat: Add CSP3 script-src-elem directive

[mark-unwin]

Added a new CSP3 directive for Content Security Policy for script-src-elem.

[mergeable]

Hi there, mark-unwin! 👋

Thank you for sending this PR!

We expect the following in all Pull Requests (PRs).

• PRs must be sent to the appropriate branch
• All git commits must be GPG-signed
• Must follow our style guide
• Be commented in the PHP source file
• Be documented in the user guide
• Be unit tested
• Pass all checks in GitHub Actions

Important

We expect all code changes or bug-fixes to be accompanied by one or more tests added to our test suite to prove the code works.

If pull requests do not comply with the above, they will likely be closed. Since we are a team of volunteers, we don't have any more time to work
on the framework than you do. Please make it as painless for your contributions to be included as possible.

See https://github.com/codeigniter4/CodeIgniter4/blob/develop/contributing/pull_request.md

Sincerely, the mergeable bot 🤖

[neznaika0]

Please do not modify the initial PR template with checkboxes.
You do not need to merge the previous comments from 4.7 - the team will do this.

[michalsn]

Please make sure each pull request only contains commits for its specific change. Commits from other, already merged PRs should not be included.

[github-actions]

👋 Hi, @mark-unwin!

We detected conflicts in your PR against the base branch 🙊
You may want to sync 🔄 your branch with upstream!

Ref: Syncing Your Branch

[paulbalandan]

Please fix the merge conflicts.

[michalsn]

Hey @mark-unwin, are you still interested in this feature? If yes, could you update this PR in the next two weeks?

[mark-unwin]

What do I need to do to have this accepted?
The commit attached 6e98ce5 is not mine, nor related.

[neznaika0]

@mark-unwin rebase:

git fetch upstream
git switch 4.7
git merge upstream/4.7
git push origin