feat(react-email): support $computed functions and custom directives in render

Closes #459

renderToHtml / renderToPlainText built the prop context with only stateModel and repeat scope, so $computed expressions and custom $-directives stayed unresolved in emails, while the same spec worked in the React renderer.

Changes

• RenderOptions gains functions and directives (same shape as the React <Renderer> props).
• The render pass carries one base context (stateModel, functions, directives) instead of just stateModel, so every element, repeat child and visible condition sees them.

Tests

• 2 new tests in render.test.tsx ($computed via functions, custom directive via directives in both HTML and plain text). Both fail without the change.
• packages/react-email: 35/35 pass, tsc --noEmit clean.

Pairs with #458: once that lands, directives also work as operands in visible conditions here.

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