fix(isHexColor): add `require_hashtag` option

[Numbers0689]

isHexColor: Enforce # prefix for Hexcolor validation

Description: This PR updates the isHexColor function in validator.js to enforce the presence of a # prefix for hex color validation. Previously, the function accepted both formats with and without #, leading to inconsistent behavior. Hex color codes are commonly expected to start with a #, so this change aligns the implementation with industry standards.

Changes Made:

• Updated the regex to require a # prefix.
• Adjusted the test cases to ensure validation only passes for hex colors with a #.
• Invalidated hex colors without # in the test suite.

• Ensures stricter validation for hex color inputs.
• Aligns with CSS and other standard hex color formats.
• Prevents unexpected behavior in downstream applications.
• ref: mdn

Checklist

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

Please let me know if further adjustments or additional tests are needed!

[WikiRik]

See my comment in https://github.com/validatorjs/validator.js/issues/2533#issuecomment-2739678804 I don't see this becoming the default behaviour, but we could add an option to enforce the use of # prefix

[Numbers0689]

thank you for the feedback! that makes sense. I'll implement the requireHashtag option as suggested to avoid any breaking changes. I'll update the pr soon.

[Numbers0689]

@WikiRik I've updated the implementation to add a require_hashtag option, updated the tests, and made corresponding changes to the readme.

[Numbers0689]

@WikiRik done!