[charts] Copy altair / vega code

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Description

Ref: https://github.com/marimo-team/marimo/issues/4585#issuecomment-2842089155

It would be useful to copy altair code based on the configuration set up in chart editor.

Suggested solution

Create a new altair code generator.

Alternative

use altair.Chart.from_json but this is not as useful

Additional context

No response