feat(isBase64): improve base64 validation based on RFC4648

[aseyfpour]

Add padding to the options list.
Update regexes to support validation with and without padding.
Update default options to maintain backward compatibility.
Add new tests to cover various scenarios.

In the previous version, only Base64 strings with padding and URL-safe Base64 strings without padding were validated. However, according to RFC4648, both standard and URL-safe Base64 variants can be valid with or without padding. Therefore, this version supports four validation combinations—standard with padding, standard without padding, URL-safe with padding, and URL-safe without padding—while maintaining backward compatibility.

Checklist

• PR contains only changes related; no stray files, etc.
• README updated (where applicable)
• Tests written (where applicable)
• References provided in PR (where applicable)

[WikiRik]

Thanks for the quick updates. I'll need to take some more time for a proper review, but on initial look this should be good so I'll request some additional reviewers already

[aseyfpour]

@rubiin @WikiRik

It’s been a month since the last changes. I’d appreciate it if you could involve another reviewer for this PR.