Add option to optionally disable webp image format

There are a number of concerns both present and looming over users concerned about the security of libwebp. Right now TBB doesn't natively disable webp in safest, so users who are on safest who do not wish to expose themselves to potential libwebp vulnerabilities need to force their accepts in order to have the same fingerprint as other 'safest' users. TBB or at least the security level should be setting the accept headers so it is more likely to be consistent amongst users. (Didn't we used to force the default ESR 52 accept headers?) I'd like to suggest that at some point disabling webp in safest be considered. The library has had major vulnerabilities in the recent past (including probably the most critical firefox vulnerabilities in the past few years in CVE-2023-4863) and it has not to my knowledge had any kind of audit since the last time a critical vulnerability was found by stumbling across exploits live in the wild. Pressingly, mozilla decided shortly after CVE-2023-4863 to remove the option to disable webp in release firefox. As far as I am aware, ESR 128 is going to include this change. Tor Browser Bundle needs to preserve the option to disable webp handling after it is migrated to ESR 128.