[css-text-decor-4] Add 'inset' keyword to text-shadow

[mozilla-apprentice]

Resolutions were made for csswg-drafts/#6074.

[css-text-decor-4] Add 'inset' keyword to text-shadow

• RESOLVED: Add inset to text-shadow
• RESOLVED: Text shadows start at outer (for outset) or inner (for inset) edge of stroke

Discussion.

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