Fix code scanning alert - Polynomial regular expression used on uncontrolled data

[Godsmith]

Tracking issue for:

• https://github.com/Ericsson/PlantUML-Interactive-Editor/security/code-scanning/99

I think this and similar security issues can be solved by using the fact that endwhile always occurs first on a line, optionally prepended by spaces, and therefore we could add "^ *" at start of the pattern. This ensures that it will only be run once per line.

I think something like the following armchair code should work:

 endwhile_text = re.sub(
        r"^( *)endwhile \([^()]*?\)",
        f"\1endwhile ({breakstatement})",
        endwhile_text,
        flags=re.DOTALL,
    )