fix(isDataURI): accept case-insensitive URI schemes

[20ns]

isDataURI('DATA:,Hello') returns false, although URI schemes are case-insensitive under RFC 3986 section 3.1. Mixed-case schemes such as DaTa:text/plain;base64,SGVsbG8= are rejected too.

Lowercase the existing five-character scheme slice before comparing it with data:. Add uppercase and mixed-case examples to the existing fixtures, including malformed inputs that must still be rejected.

Validation:

• The added regression fixtures fail on the unchanged source and pass with the fix.
• Node 24: npm test passes, including all builds, lint, and 323 tests.
• Node 22: npm run test:ci passes, 323 tests.
• 560 additional assertions pass across source, CommonJS, ES module, browser, and minified browser forms, covering all 16 case combinations of data with valid and invalid inputs.
• 100% line and statement coverage; git diff --check passes.

Checklist

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

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (9ff3424) to head (89e9aac).

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #2893   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          114       114           
  Lines         2599      2599           
  Branches       658       658           
=========================================
  Hits          2599      2599           

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