65417 Editor: Recognize uppercase URL schemes in link validation.

[nimesh-xecurify]

Updates the regex used in the link dialog to support schemes with uppercase letters. This prevents the editor from prepending http:// to URLs like HTTPS:// or Http://, which are commonly produced by mobile keyboards.

Includes new QUnit tests to verify correct handling of uppercase, mixed-case, fragment, and root-relative URLs.

Trac ticket: https://core.trac.wordpress.org/ticket/65417

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