Disallow isolate Unicode characters in comments and strings

[cameel]

Part of #10254.

Description

As shown in Security advisory for rustc (CVE-2021-42574), the isolate Unicode characters (LRI, RLI, FSI, PDI) can be used to take bit of text out of context and show it at the beginning or end of the line. We should disallow them to prevent this.

We have already disallowed unbalanced BiDi embedding/override characters in #10326. This is unfortunately not effective against the trick described here.

See How to use Unicode controls for bidi text for details on how these characters work.

[cameel]

Actually, we need to consider whether just going for #10607 is not a better solution than playing whack-a-mole with individual Unicode characters.

[github-actions]

This issue has been marked as stale due to inactivity for the last 90 days.
It will be automatically closed in 7 days.

[github-actions]

Hi everyone! This issue has been automatically closed due to inactivity.
If you think this issue is still relevant in the latest Solidity version and you have something to contribute, feel free to reopen.
However, unless the issue is a concrete proposal that can be implemented, we recommend starting a language discussion on the forum instead.

[ekpyron]

Keeping this open, since this is not going away - even though in practice we may fix it via #10607