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GH-30863: [JS] Use a singleton StructRow proxy handler (#44289)

### Rationale for this change

Fixes #30863 by using a singleton proxy handler in `StructRow`'s
constructor. Since the handler is stateless, there is no need to create
a new instance for each row.

### What changes are included in this PR?

Refactoring `StructRow`'s constructor to extract the proxy handler.

### Are these changes tested?

No additional tests since this is an internal refactoring, but `yarn
test` runs successfully.

### Are there any user-facing changes?

No.

* GitHub Issue: #30863