Add layout expressions for view positions

[ibgreen-openai]

It would be very powerful to be able to use CSS style calc expressions when declaratively specifying view layouts.

Summary

• replace the simple position struct with a LayoutExpression tree plus parser and evaluator so strings with numbers, percentages, px units, and calc() expressions can be represented and evaluated
• store the parsed layout expressions on View and evaluate them when resolving viewport dimensions and padding
• add tape tests that cover literal numbers, percentages, pixels, and calc() combinations as well as direct evaluation of synthetic expression trees

Testing

• NODE_ENV=test node node_modules/@vis.gl/dev-tools/dist/test.js node

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https://chatgpt.com/codex/tasks/task_i_68fb7cdcc1e8832ca8367f01290541fc