feat: add createUrlTransform factory for custom safe protocols

[Aimee1608]

Initial checklist

• I read the support docs
• I read the contributing guide
• I agree to follow the code of conduct
• I searched issues and discussions and could not find anything (or linked relevant results below)
• I made sure the docs are up to date
• I included tests (or that is not needed)

Description of changes

Summary

Adds a createUrlTransform(safeProtocol?) factory function that returns a URL transform function with a custom set of allowed protocols, without requiring callers to reimplement the full sanitization logic.

Motivation

Currently, extending the list of safe protocols (e.g. adding tel:) requires fully reimplementing defaultUrlTransform, which means callers lose the benefit of any future improvements to that function (see #941).

Changes

• lib/index.js: Extract the URL sanitization logic into createUrlTransform(safeProtocol?) and redefine defaultUrlTransform as createUrlTransform() — fully backward-compatible.
• index.js: Export createUrlTransform from the package entry point.
• test.jsx: Add a createUrlTransform test suite covering default behaviour, custom protocols, blocked protocols, component integration, and parity with defaultUrlTransform.
• readme.md: Document createUrlTransform with a usage example.

Usage example

import Markdown, {createUrlTransform} from 'react-markdown'

// Allow tel: in addition to the default protocols
const urlTransform = createUrlTransform(/^(https?|ircs?|mailto|xmpp|tel)$/i)

<Markdown urlTransform={urlTransform}>
  {'[Call us](tel:+1-555-0100)'}
</Markdown>

Backward compatibility

defaultUrlTransform behaviour is unchanged — it is now defined as createUrlTransform() with the same default regex.

Closes #941

[JounQin]

kind of duplicate of #942

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (fda7fa5) to head (1f9b88d).

Additional details and impacted files

@@            Coverage Diff             @@
##              main      #943    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files            3         3            
  Lines         1743      1862   +119     
  Branches       123       129     +6     
==========================================
+ Hits          1743      1862   +119     

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