Fix highlight sc so that it removes shortcode offset from inner code

[kaushalmodi]

Fixes #4717.

[kaushalmodi]

@bep When you get a chance please review this PR.

• It passes all the existing tests.
• It passes the new test added specifically to test to fix. I have verified locally that if I revert the fix, this new test starts failing.

[bep]

@kaushalmodi I appreciate the effort, but

1. I need to understand this problem, which is a post-Christmas activity
2. We cannot have logic at this level in the shortcode templates. If so, that would have required a different level of test coverage

In the mean time, I suggest you put this shortcode inside your own project(s).

[github-actions]

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