Add struct/row proxy objects via useProxy option.

[jheer]

• Adds an option for representing Struct types and table row objects as zero-copy proxy objects with getter properties that retrieve data directly from underlying Arrow batches. This representation is enabled by passing a useProxy option to table generation methods. These proxies require substantially less memory and are faster for single-access use, but do not support object manipulation methods such as Object.keys, Object.values, and spreading { ...object }.

The proxy representation follows the strategy of the Vega arrow loader and suggestions from @mcovalt.

Benchmarks show non-trivial performance improvements (25-35% reduced running time) for these proxy objects vs. vanilla JS objects: 26.72ms vs. 40.23ms for flights.arrows and 441.24ms vs. 576.23ms for scrabble.arrows. However, due to their simplicity and convenience methods, this PR leaves vanilla objects as the default object representation -- hence the opt-in via useProxy. Both methods in Flechette remain substantially faster than the Arrow-JS reference implementation.