Fix case sensitive linting

[Kraemii]

What:
This PR reverts some changes from #728, to be exact the changes from the commit f123713.
SC-584

Why:
The nasl linter was not able to detect undeclared variables in case of different letter cases, e.g. for the linter cpe and CPE was the same variable name even though it is not.

How:
Run openvas-nasl-lint test.nasl before and after with this small script:

CPE = "foo";
display(cpe);
exit(0);

Checklist:

• Tests
• PR merge commit message adjusted