Editor: Add ARIA labels to LinkControl dialog

[Zaidfarooqui01]

Gutenberg/WordPress Core: Editor

Add accessibility improvements per Trac #64490:

• Add role="dialog", aria-labelledby="link-dialog-title", and aria-modal="true" to LinkControl modal container
• Add hidden heading id="link-dialog-title" with text "Insert Link"
• Add aria-label="Link URL" to URLInput field
• Add aria-label="Search links" to search input field

Tested: Chrome Accessibility Tree + NVDA screen reader

Fixes #64490

Props: mohammadzaid

See: https://core.trac.wordpress.org/ticket/64490

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Props mohammadzaid.

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[t-hamano]

Thanks for the PR!

Unfortunately, this PR isn't acceptable because the LinkControl component is managed at the Gutenberg repo. See https://core.trac.wordpress.org/ticket/64490#comment:2